Bet a cold adult beverage that the real reason is the same one that got SecArmy canned a few years ago: going behind OSD’s back to lobby Congress for more stuff. (Crusader for the Army, F22s for the Air Force)
As I recall, GEN Shinseki was canned because he publicly disagreed with Rumsfeld on the number of troops to use to take, hold, and pacify Iraq.
Looks like Shinseki was right, too. Oops.