The Vet told me it was because he was straining when trying to pass a poo, I haven't seen them trying to mount each other but you can't watch them all the time, I have feed him tonight and touch wood it seems to have gone back in hope it stays in tonight, the vet did tell me once his 21days movement restriction is up due to me worming them last week even though he not up to weight to have him slaughtered and use him for my own consumption, to keep him to finishing weight would be a worry and a waste of food really as I would be constantly waiting for it to happen again. I fear that once it has happened the vet said there is a good chance it will continue to happen.