I think there is something about sparing with another cock for territorial rights that gets satisfied. If he couldn't get to the other guy the fight gets transferred to whoever is handy. It may be possible that my guy took out his frustrations on his favorite hen. They roost together at night...
I have an aggressive cock right now I am dealing with. A couple of weeks ago he was trying to climb through the pen fencing trying to get to the lawn mower. He tore up a hen so I put him in time-out. He came back and still attacked the hen, (one of three Silver Pied hens). Now on of the hens...
It is aggression and frustration that causes this behavior, usually from other outside influences. We normally see this in penned birds where there is something outside the pen that the cock can not chase off from his territory. Your guy is free-range but he could still be frustrated by some...