The opinion of the M.B.C.I.D. Model Boat Crash Investigation Department.
Is while accelerating with the waves and bouncing quite a bit, the boat operator opened the throttle hard, the resulting torgue twisted the boat sharply and flipped it upside down.
Luckily the camera operator BEC was quick enough to get a shot of the action.
The reason the stern is so low under the surface is the prop is still under power and pushing it down very hard.