It is called an elevator accelerated spin, and it happens because of the conservation of angular momentum as stated above. It's easiest to think of the engine swinging around up front. When you push foreward the engine is brought closer in to the axis of rotation and the result is a faster rate of spin. It's actually a bigger killer in the full size world then the famed flat spins, because you are doing one of the correct recovery inputs, just not in the correct order.

Ty