After I shot the deer, I tied up the horse, cleaned the deer, then packed it on the horse and headed up the mountain, leading him.
Coming up over the top of the mountain the horse suddenly sat back against the lead rope, then began to jerk, kick and run backwards. I tried to get around behind him and whack him with a rope to take his mind off of whatever was bothering him. It did no good. I even fired a couple shots into the ground behind him, but he continued to run backwards until he fetched up against a large Mesquite tree. By this time my pants leg was shredded, the deer was hanging upside down underneath the wall-eyed horse, and everything had gone to heck. After finding he could back up no further the horse settled down and soon I was able to get everything off him and lead him out to a clear spot. I tied him to a tree got everything repacked. After that episode we had no more problems and made it home without further incident.
All I could figure is maybe his ears stopped ringing and he suddenly woke up to the fact there was a dead creature on his back. Who knows?