Lesser Nobles 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 is the risk of being declared an outlaw by the king, and the intervention of neighboring provincial overlords or even the kings 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. Only the king and parliament can remove the title (duke, count, earl) of a Great Noble.