Since we can only have a lot of fun speculating (and boy, is it fun!), my money is on a family member or close friend. I don’t see any way all of those injuries are from a fall... the scene would be like a Roadrunner cartoon. And only a very close friend or family member would be able to get close enough to him (security detail) to do the damage. This man is loathed by many so I have to assume security is fairly robust.
Since I am in “Godfather” mode, I like to think it was mob-related and Larry had more stones than Fredo.