I'm very sorry for your loss/your sister's loss. If you really want to know answers to cause of death, you could have a necropsy done, but they don't always tell you that much.
My initial question to the chewing was also could it have been post mortem?
However, rabbits will engage in self-mutilation in the case of nerve damage, such as sciatic nerve damage. Could he have fallen, or gotten a leg stuck somewhere and then twisted it? I suppose it could be a possibility, but it's really hard to say.
No matter what the reason, I'm really sorry that the poor bun went through it and that someone had to find him like that. It must have broken your poor sister's heart to see that.