I am working on an encounter inspired by Stardust and struggling with a central question I hope you can all help me with :) 

So... a star has fallen and the party is hired to find and hide the star, not knowing that what they will find is a living person and not a big lump of rock. 
Like the movie, there are a lot of people who would like to get to her as eating her heart can prolong your life to last for centuries. 

I've pretty much got it all planned out, but one question remains: 
Why/how did she fall from the sky? 

In Stardust, Evain was "Knocked out of the sky" by the king's ruby, but that is very circumstantial to the storyline in Stardust. 

Leaning in on the idea the star is a living being, perhaps they saw something that piqued their curiosity. When they approached for a closer look, they discovered gravity was more powerful than they anticipated and were trapped.

P.S. Yay for Stardust!

Kicked out of the sky by some celestial conflict

Pulled down by someone in the lands below

Drunk-driving spelljammers :)

Got curious about something below, and came down deliberately

Somebody down below cast wish (That's how it works, right?)

Came down seeking allies for some sort of star.. war?

How about a sinister angle? There's something up there eating stars, and this was the only way to escape!

A moving Dead Magic Zone disrupted the star's suspension.

A lost bet by a solar resulted in sending the star down back from the sky.

A celestial bard drop the mic after a stellar performance.

The term came to an end, ceasing any space activity.

An adventuring party on the ground pulled a lever they weren't supposed to.

A reverse gravity as a result from a Wild Magic Surge miscast by a spelljammer sorcerer.

(Love this encounter, will definitely steal.)

- Star is clumsy and she tripped on some cosmic radiation.

- Falling stars are just celestial kids on a gap (light)year exploring the universe before they decide whether or not to turn nova or settle down and build a solar system.

- Star is in love with a terrestrial and fell to be with them. (Literal star-crossed lovers.)

- Stars feed on mortal wishes. Nobody has wished on her in ages, and she's weak and seeking help. 

- Star came down on a dare.

- A cosmic horror, the Consuming Dark, is eating all light in the outer reaches of space. She fled to survive and fears going home. 

- She was a human cursed to be a star by some hag centuries ago. An adventuring party has just killed the hag. 

Space Clowns. D&D solar systems are surrounded by an airless vacuum which is enclosed by a Crystal Sphere. Inside the sphere is Wildspace. Outside is the phlogiston. 

Space Clowns are descendants of people who really picked the wrong deity to worship. They are cannibals. Their home Wildspace is know as Clownspace from which they travel via the phlogiston to maraud through other Wildspaces, sometimes landing on planets and setting up carnival tents to lure in their meals. The "Star" is a victim who escaped one of their raids. Since the phlogiston is between all Crystal Spheres it allows the Star to be anyone or anything you wish. The "shooting star" being the protection the Star was using burning away. 

an intelligent murder comet batted aside during an astral drifter's brush with divinity. the casual flick from a god has cursed the ancient wizard spirt with renewed mortality in a fresh new body (and far from their now ancient resources). there's a lot the disarmed wizard can reveal about spelljammers, the deck of many things in general, and the the Donjon sphere in specific. but will they be telling your party or will the creepy, mysterious Heralds of the Comet get there first? and when a criminal gang, the Moonstalkers, offer to intercede, are their motivations as obvious as just the favor you'll owe?

Or...you could keep the canon which exists in Forgotten Realms about the Far Realm-infested stars of Realmspace. They are stars which have been overtaken by entities and can affect the universe in strange and eldritch ways. It already sets a precedent that stars can contain sentient entities. Maybe your star is fleeing a more powerful malevolence that was attempting to consume its power, corrupt its essence, or in some way force it to act against its nature. It could flee/fall to a safe, magic rich location out of desperation, triggering your story with an unnamed mechanization driving the narrative to be revealed later. Hadar and Acamar are great choices for hungry entities which can empower your antagonists and shape their hunt.

I will include the Wiki link here and hope it is still within the guidelines of this site.

It's possible that the star made it to the Milky Way to see the light's all faded and that heaven is overrated!

In order to shine, she needs to consume souls. She will play the week innocent in need of aid while people come to hunt her. Every hunter they kill gives her a soul to feed upon.

She got curious about life on the planet and leaned down to get closer look, but lost her balance and fell to the ground.

So, in my cosmology, the stars are literally the resting places of heroes of yore, placed in the Firmament by the hand of a goddess.

In my case, one would simply "come loose" -- and there are so many stars, that the stories of those who are within them are not always known by those living today...

Also, I had totally forgotten about stardust, and so thank you for reminding me -- it will make a wonderful sidequest.

Leave that as the unanswered question. Let the players run wild with what they think happened......Then to be a real pain. Never reveal the answer. Somethings in life we just never will know.

i agree with LordVivianVader that keeping it unanswered is probably the most interesting option  but maybe in the future of the campaign you can recall to this event. and they could learn some theories from a scholar or a library of astronomical knowledge.

Quote from LordVivianVader >>

Leave that as the unanswered question. Let the players run wild with what they think happened......Then to be a real pain. Never reveal the answer. Somethings in life we just never will know.

 This is the most solid answer TBH. Players make up all sorts of cool stuff in game. As a DM, if you just let them chat in character, they will probably come up with some great ideas instead of you feeling forced to shoehorn something into your narrative that they may misinterpret and create a frustrating situation for your story. I have run so many games where I had an idea about something, then the players read the situation differently and I just went with the side-quest and folded it into the narrative without the players knowing I was running by the seat of my pants just lke them. It can really push your storytelling and player management skills, but it makes the whole game even more exciting for you as the DM, in my opinion.

Well, "stars" don't fall from the sky.  Meteors and meteorites "appear" to fall from the sky, but they just fall from orbit or near orbit.

So let's roll with the idea that folks think a star fell from the sky, but it was a meteor.  If the person of interest doesn't have to have fallen to earth with the star, they could just be in coincidental proximity.  Now there are a ton of D&D reasons this could be so.

A godlike figure could have wished the person to be discovered and associated with the star.  A powerful magic user could have caused the meteor to fall, and teleported the person to near the site a few moments later in order to be discovered and associated with the event.  An astute astronomer could have seen the meteor and predicted it would fall, and someone in his circle of friends wished for the person to appear "with" the star.  So, with magic and god-like beings interested in the Material Plane loads of things could happen that appear to be a star falling to earth and a person is delivered with the star.

With a good Mind-wipe spell, maybe the adventure could be helping this person reveal where they came from and who they really are.  Maybe they arrived at the impact zone because another spell was interrupted.

Quote from MetteM >>

I am working on an encounter inspired by Stardust and struggling with a central question I hope you can all help me with :) 

So... a star has fallen and the party is hired to find and hide the star, not knowing that what they will find is a living person and not a big lump of rock. 
Like the movie, there are a lot of people who would like to get to her as eating her heart can prolong your life to last for centuries. 

I've pretty much got it all planned out, but one question remains: 
Why/how did she fall from the sky? 

In Stardust, Evain was "Knocked out of the sky" by the king's ruby, but that is very circumstantial to the storyline in Stardust. 

 In the movie the reason is basically fate. Her falling sets off a chain of events that fulfills the wish of crowning the rightful king.  Star dust is using stars  like in astrology where they are tied to ideas like fate. 

• They fell to fulfill some wish.
• They  fell as part of a prophecy or fated sequence of events 

The former would have a tangible character who made a wish , the later requires only that their presence lead to some grand event. 

The fated answer can also have a seemingly coincidental actual cause  e.g

• Tripping over 
• getting knocked out of the sky 
• coming down for a visit 

As other people have mentioned the stars are also tied to ideas of divinity and  the unknown. Though if they are being rescued I don't know how those kinds of forces will work with that. 

Star got hit with the Wave Motion Gun.

the elder evils look like stars, perhaps the star inst a star at all, but an elder evil who came to earth purpousely to start some kind of cult, and took on human form