Lesser Nobles should appeal to the King to replace their superior (a Great Noble). Naturally, this will upset the Great Noble, but his nobles are obviously organized against him in such a situation. What dissuades Lesser Nobles from simply marching on their Great Lord's fief and killing him outright is the risk of being declared outlaw , as well as the probable intervention of neighboring Provincial Overlords, or even the King's army. By setting their case before the king, they have a chance of getting their Overlord replaced without the bloodshed and expense of a rebellion.