I'm in the process of creating my wood elf druid character's backstory. I need some ideas on how the city she grew up can later turn out to be corrupt. Please share any ideas you might have or anything you may have come across that might be useful! Many thanks guys 

What do you mean by corrupt? As in the local law being corrupt? 

Do you mean that the land has been corrupted? 

If you are talking about politically, economically or socially corrupt then here are some ideas.

Politically:

The leaders are seeking to advance themselves on the backs of the middle and lower classes.  They over tax the people.  They have select insiders that are allowed to get away with crimes, including high-end stuff like seizing land through fraud.  They use the money to influence other higher political figures so they can be advanced.  Or they use the money to fund an army that expands their holdings seizing the assets of other free peoples.

Economically:

The merchants have established a series of monopolies and a scheme of price fixing.  The people have to pay excessive costs for normal items used in their day-to-day lives.  They receive less than fair market value for their crops and livestock.  They are unable to establish trade through other means to deal with outside suppliers because of bandits and/or other merchants don't want to risk sending goods to that area for fear of reprisals.

Socially:

The leaders in town have decided who is "in" and who is not.  Access to important individuals / markets is restricted to members of the "in" crowd.  Anyone trying to present themselves as members of the "in" crowd by dressing in the proper fashion, buying the right things and speaking to the right people are sent a message that they better cut it out or else.  Members of the "in" crowd largely ignore folks they don't wish to speak to.  There are shady characters and rumors involved in what happens to someone that crosses the power brokers in society.

I’m looking for something that would be only known to a few of the leaders of the city. It needs to be enough to cause some like of revolution within the city later on when people find out if that makes sense? 

In that case the classic is: leaders are possessed or mind controlled, and have an evil plan to kill all people, rob their children or establish one of the brilliant examples of MusicScout. :)

Known by a few people and would cause a revolution if word got out?

How about something like: An overpopulation problem solved by Soylent Green. (If you discovered you were eating your neighbor who had gone missing a week ago...)

See Baldur's Gate in Descent Into Avernus