This has been speculated on repeatedly. It's likely due to excessive g-force, vibration, or both. The consensus is the tailwheel lock was broken in the pitch-up. As for the missing pilot, either he's hunched over the controls in the front or the seat broke and he's all the way back. The g-force in the pitch-up was extreme (9+), and probably caused a blackout. The force might even have broken the seat. The aircraft rolled into a inverted dive in what looks like a torque roll - rotating around engine torque from the ultrapowerful modified engine. That may not have been a controlled maneuver.
I am not a high-performance piston pilot, and I don't understand nor would I speculate about what actions a pilot might take to recover from this condition.
Just to add a little perspective - I have been on-board for many flutter test flights - thankfully all of which proved the aircraft did not have a flutter problem. However a fellow I know was on-board a business jet undergoing testing that DID encounter flutter. The control wheel was ripped out of the pilots hand, and the control column vibrated back and forth at such a rate that it was a blur and neither of them could grab it. A flutter event is violent, and often shakes the aircraft apart. There is some famous footage of a stealth fighter that had aileron flutter and it tore the wing off.
Your description sounds very plausible. I had not seen that explanation before. Thanks.