It\'s called a "tank-slapper" It happens in all forms of powered motorsport, when a vehicle as travelling faster than the tyres can handle, they start to slip, then the driver instinctively tries to rescue the situation by steering with the skid. As they do so, they become more unstable as the vehicle increases side to side momentum. I think the term "tank-slapper" has something to do with when the fuel tanks are half empty/full. The sideways centrifugal movement of the fuel in the tank (especially in older bikes), amplifies the instability of the bike when this sort of slipping occurs.