I will also say rut related. Rut can be tough on some buck. But my thoughts are more that he might have been stuck with an antler somewhere that we can't see. Seen bucks get an antler tip in the underside of the belly/flank area that you could not hardly see until you shot them. That type of wound is very common in bucks after they loose a fight. The winning buck will gore the looser as the looser turns to flee. They catch an antler in the gut or in the butt. He also appears to be missing his tail.
So you're saying is, he's had his butt tore up...