I have no 1st hand info. I heard a rumour that the bikes were in the accident together. That could explain the culpable homicide case.
Since I don't KNOW this for sure, I regard it to be just that. A rumour.
What is clear is that BMW do not want to make more info available. Some people on this thread spoke to people directly involved and they don't want to elaborate or even tell what they've heard. I do not know any specifics or the reason's for the secrecy, but the speculation will not stop. It's natural for people to wonder what happened, especially if it's being kept a secret.
It's a pity because at first it was said to be "out of respect for the family", but now it hurts more than it helps I'm afraid.