Best explanation i've heard is that when people get shot in the head, they sometimes "twitch" or "spasm" violently. I believe years ago I saw some documentary which stated this was the cause of his backward motion.
Besides, entrance and exit holes indicate direction of travel. Little hole for entrance, big hole for exit is the norm.
But there WAS a big hole. In the BACK of JFK’S head. According to the person from the Dallas funeral home employ who was tasked with covering the ENTRANCE ( small) wound and the back of the skull wound that he stated was as big as a fist. If Mewsmax ever shows The Men Who Killed Again, it would be well worth watching