I am a DM in my second campaign. And the first one without a pre made story. The campaign is in the Planescape setting. although we don't run the "turn of the fortune wheel" campaign, I want to use the multiversal glitch feature. For this I need an Idea for the cause of the glitch. I don't want to use the one from the book, because it would lead to using too much pre made stuff.

The Players don't know of the glitch yet, because no one died, until now. And I want to reveal it in play.

I have an Idea myself, but I am not entirely convinced with it, yet. My Idea ist:

Some kind of artificer or artificer like person tried to build a device, to control the tides of the multiverse. But upon activating it, it malfunctions. Which causes the multiversal Glitch. Maybe the PCs have incarnations which where helpers to the artificer, to explain why they are affected by the glitch.

Maybe I could let the Players discover the device early in campaign, so can decide themself, If they want to let the glitch continue. But when they do, later in the campaign, some kind of even happens to force them to decide to repair ( hard way) or destroy the machine.

Also while writing this, I thought, that the artificer could be the BBEG, who is also affected by the glitch, so he returns after each death in a new Form, with new a way of making their live harder.

I don't have a good suggestion.

However, you don't actually need to know right now. You are allowed to introduce the problem, and figure out the cause later. (Just be careful not to contradict anything you do reveal to them about it.)

One classic DM trick with the right players is to listen to them speculate about something, and take the answer from their speculations. It makes them feel clever.

The Multiversal Mushroom was taken from its sacred grotto at the heart of The Spire by a mischevious tabaxi, and the multiverse has been destabilized. Not only are the players reincarnating, but now everywhere is a wild magic zone that not only triggers when someone casts a spell but also when someone rolls a nat 20.

I have a few

1. Time travel 
2. Trying to drill outside of the planes to a completely different reality with different laws of physics and planes
3. Trying to make a new plane with primordial energy of creation
4. Trying to destroy a plane
5. Just a plane old bid for immortality, warping reality to enforce his existence as a rule 

howabout.... some kind of high magic fu*kerry has turned the great wheel, causing some kind of massive upheaval, the perfect cover for a crime against reality!

Thank you, for your suggestions, I will try to come Up with a cool reason for the glitch.

I'm late for the party, but what about a Lich phylactery gone wrong? A Lich on some world is attempting to ascend to godhood via an advanced phylactery that pulls in souls from across the multiverse. This is causing instabilities and glitches as deaths are interrupted (souls pulled into the phylactery instead of the afterlife). But... it's pulling every possible incarnation of a creature, one death at a time, instead of randomness.