Racoons have been known to reach through a fence or hole and grab the chicken by the head and remove it. Don't know why. Other animals will eat only the insides of the chicken and not the meat. Guess it depends on what they hungry for
By just eating the head off, to me it sounds like an animal that is mainly out for a quick meal of blood. The head is the closest to the juglar veins and to remove it gives the quickest source of a blood meal. Hope you're able to trap the varmit causing you this grief.