At a guess, things that they could refuse to payout on related to motorbike accidents:
No helmet.
Intoxicated (had been drinking or high) at the time of the accident (according to a medic).
Riding a motorcycle for which you're not licenced (Riding a 750cc or 1,000cc bike when your licence only covers you up to 125cc).
In my many years in Thailand I almost always managed to avoid riding on motorbikes anyway, too risky.