Your bike is leant over, usually going round a bend, back tyre loses grip and skids, thats ok, just brings the rear of the bike round a little, then the back tyre suddenly finds grip, this has effect of sitting the bike up suddenly and can 'flick' the rider of the seat in the oposite direction of the lean. Dpending on the speed he will either land on the seat again, or may fly completely off the bike.