My Theory, absent an official report on cause
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. He probably had his harness off to reach something in the cabin... fire extinguisher, charts, loose bag, sunglasses, who knows. I asked Gary about the safety of having the doors off, as he often did. He told me the door latches were extremely weak, and they would not hold you in if you lurched into the door. He said it wasn't important because you always had your harness on. If the rotor seized, it wouldn't have mattered anyway, because helicopters don't fly with seized rotors, and 800 feet is too high to survive. I didn't see any reports about whether the rotor was turning after the "explosion". I would be very interested.