My theory is that the main rotor gearbox or the engine seized. This would have twisted the cabin violently to the right, throwing the pilot against his door.
I'm not so sure the cabin would go to the right. If the engine seized, the sprag clutch would allow the blades to freewheel. If the gearbox or the output shaft bearings seized, the friction created from the inertia of the blades would cause the cabin to go to the left. (try applying the rotor brake whilst shutting down floating on floats will demonstrate this).