Fall of rider. Horse out of control. Horse in charge of situation, not competitor. Small child being walked over by horse they are unable to control. Emergency dismount completed safely.
Parent runs in make sure they are all right.
I define unauthorized assistance as ANY help, whether requested or gratuitously offered, which can affect the outcome of the activity. The point is that since the rider can not continue anyway - no help they receive will improve their score. The hard case comes from unsolicited advice. If the rider was really going the wrong way, they may be in trouble whether they knew the yeller or not. If they were going the correct direction, did not appear lost, and made no acknowledgement of the advice, I may well give them the benefit of the doubt.