I guess It has something to do with the amount of nobles joining a particular side. (only reasonable thing i can think of)
Rebellions are rare though and the only ones i actually witnessed where more like as Chenier describes: with a general or Duke disbanding all Militia.
A well planned rebellion by a General can easily be won by ordering all nobles far off(except he and his buddies), disbanding all militia and just wait for a day in the capital.
