My coop is a fort Knox, the danger came from with in. Hubby said he saw the Buff Orp have a go at her with a couple others to but off and on. He did not realize the severity of the matter. I know it only takes a peck to draw blood then it's on with chickens. It just was such a shock as they have never done this before. They all seem calm as if nothing has happened.Myself, I have never had good luck with joint issues. Those I always cull, as it looks painful and they rarely, if ever, recover from such a injury. Maybe something tried to pull it through a fence, like a coon. You might want to put some hardware cloth around the coop and put out a trap.