Check the pivot point of the idler belt tension bracket for wear. If the pivot point is worn too bad it will allow the idler pulley to be out of alignment with the other pulleys and cause the belt to come off. That pivot mounting point takes a real beating so with the belt off raise the pulley/bracket up and down to check for loose. OR, when the belt is on and PTO engaged, kill the engine then look at the belt / pulley alignment with PTO engaged and tension remaining on the belt.
