A theory I found on line:
http://www.whitetailstewards.com/articlesonsite/deerbiology/antlerabnormalities.htm
Body Injuries (this guy had an injury to his right leg)
Body injuries caused by disease, vehicles, bullets, arrows, snakebite, and other unfortunate events can cause abnormal antler growth. Injuries to a front limb (foot, leg, shoulder) may affect the antler on the injured side, opposite side or both sides, but the antler on the injured side is typically most affected. Hind limb injuries usually affect the opposite antler. Body injuries can affect antler growth on both sides and may cause the antlers to stop growing entirely. Depending on location and severity of the injury, the abnormality may occur on just the current set of antlers or it may be carried throughout life. For example, a buck that sustains minor injuries to his right rear leg from a vehicle collision may have an abnormal left antler for one season. Another buck that loses his right rear foot to a bullet may have an abnormal left antler for the rest of his life.