It is the harmonic balance being interrupted where the weight is removed from the front wheel. It is usually associated with improper front suspension setup. Soft rubber, too much rake, worn out springs can do it to a conventional bike.
In racing is comes when a rider leaves a curve and hits the throttle hard. When the front lifts and the bike is not all the way straight all the energy is turned into a wobble. The worst thing he can do is hit the front brake. Holding on and tapping the rear brake to take off speed is his only hope. Most end badly.
It is highly unlikely a modern cruiser could ever suffer from a slapper. They don't have that type of balance and surely have no horses to overpower the front end.
