Strange Horse Injury

This post should maybe be marked NSFW because it involves a bizarre horse injury. I am curious to get some thoughts from people with horse experience.

I am not a horse owner. A family member of mine has two horses. Both females.

One of the horses was found one morning with blood running down her rear legs from her vagina. The vet found internal damage. There was a tear in her vaginal wall and active bleeding. Later they found injuries to her bladder and a pelvic fracture.

No one has been able to determine the cause of the injury. The vet compared her internal injuries to being hit by a truck.

She was in a round pin overnight. No one has been able to find any object that she could have rubbed against accidentally.

She has no external damage (scratches, etc.) everything is internal.

Does any one have any ideas? Could this even happen "accidentally" or did someone or something have to cause these injuries?

I apologize if this is not the right subreddit for this question.