The way I see it is that, in a big empire, virtually everyone is on the make, especially so if not under the close watch of the ruler.

Every General lives to prove their bravery, personal fighting qualities and/or tactical and strategic expertise to all and sundry. So being left in charge of a dull garrison duty in some god-forsaken province, remote from where they were born or where their family still live and, more importantly still, out of the king's sight so that possibilities to gain fame and favour for themselves, or influence politics and goings on at court are virtually nil, is bound to inspire resentment in some - to the point of disloyalty.

With a large enough army under their command, some would seek to set themselves up as a petty warlord (the Shogun scenario) and carve out a small kingdom in their own right, so they turn rebel. Some think bigger-scale and take the time and trouble to see who they can convince to come with them on the enterprise and, when they judge the time to be right, start a civil war, in which they hope to prove themselves. Even if they lose, at least they get to do the thing in life they trained for and also get their name in the history books. These types no doubt see infamy as preferable to non-entity.