@screamdoom
... That's all it takes. Class warfare. Economic imbalance. Oppression of the masses by a wealthy few. Look how many times in our own history that peasants have rebelled, or the working class united and rose up. Look into the history of the guillotine.
Land-owning nobles are essentially just minor kings. They can do whatever they want so long as it doesn't piss off the king/he doesn't learn of it in an age where all messages had to be written-down and hand-delivered, or kept in someone's head who might take weeks if not a month or more to reach the king. If not waylaid, or dying of disease or infected wounds or bad humors or just because.
So, someone born with the notion that they are nigh-absolute power because they're better than everyone else GIVEN that nigh-absolute power of their peerage, can treat fellow humans as scum and toys, and eventually that kind of thing reaches a point where no one can take it anymore. Modern science shows that people with too much wealth and influence, especially those raised in it, are divorced from the human condition, and have problems seeing other people as the same sort of creature that they are.