I would guess that one hen was killed at first, the others went into hiding, came back to roost after the coop was shut, and then got blocked outside and killed that night. A few days ago I was butchering my meat birds, one of my rooster came around the corner and saw what was going on, he took off into the woods and did not return for 2 days. If a chicken gets scared enough it will get out of there. The chewed on heads and neck sound like a raccoon.