for an incident with a force strong enuf to pitch a rider over the bars, i would normally assume a lock-up at the front .
however, it's not inconceivable that the culprit might possibly have it's source at the rear wheel . perhaps his freewheel/freehub ratchet pawls aren't functioning properly ,causing the chain to wrap itself around the cog and bind (ie- " chain suck ")?- the result would be an almost instantaneous & violent lock up of the rear.if his speed were high enuf(~ 20mph definately qualifies), the accident might happen so quickly that it wouldn't be recognised as a being initiated by a skid. an observer would probably not catch the first milliseconds of the accident,and only see it's aftermath.