I think the logic is they collect men until the AI thinks they can capture the target province but why they are not restricted in movement like human crusades, i.e. actally move towards the target rather than back and forth in the wrong direction. IMHO they should move directly to the target and keepattacking the province depite the odds being against them. Too often they sit on a hevily defended border for 20 years till they die a natuaral death.