OP excuse me if I'm way off the mark here...just when you said they're saying its a chicken or the egg situation; it made me see your question from another perspective.
are you asking "why did this happen" from the perspective that perhaps you blacked out prior to the accident? that is, you lost consciousness (for whatever reason) then proceeded to lose control of the motorbike, hence the accident? is this what the doctors might mean by they don't know which came first, the loss of consciousness or the accident?
I hope I'm not way off the mark, or adding to the confusion of the situation.