My limited knowledge of history suggests that it's always something really random that finally sets off a revolution. Real "straw that broke the camel's back" stuff. For example, Mohamed Bouazizi sets himself on fire and suddenly the entire Middle East is experiencing the Arab Spring. There was a lot of discontent leading up to it, but in the grand scheme of things, nobody could have predicted that that would be the final straw.

To borrow some terminology from Criminal Minds, there's a difference between a stressor and a trigger.