OK, I'll bite. I can visualize the torsional energy thing. But when the brakes are released the torsional energy would actually make the aircraft go forward until it is dissipated, not backwards. So either its the rocking out of snow effect (where going forward against an incline makes it easier to go backward etc.) or its more simple - its on a wicked slope and they forgot to chock the airplane. I go for simple.
P