Faulkner goes to the doctor

Faulkner, my new Blue-Faced Leicester ram, had a large swelling (about the size of a grapefruit and with no visible injury) behind his rib cage. I noticed it on the weekend and my veterinarian looked at him on Monday. The vet thought that she could feel an opening in the muscle tissue and recommended that I take him to the UC Davis VMTH to surgically repair it. Sure enough, there was a 2" opening that required a much larger opening by the time it was done.

So what could cause this hernia? My vet and I had a talk about what she calls "veterinary practice builders", one of which could be this:

Hudson and Faulkner have not been fighting, but it is my vet's opinion that just a quick toss of this head could cause damage to someone else.  Most of my other 4-horn rams have had curved top horns so I wonder if injuries are more common when a ram has a set of spears on his head.