Correct me if I'm wrong as I know nothing about P51's but apparently this kind of accident had happened before. The trim tab comes off and pitches the aircraft up into an extremely high G maneuver. This is obviously not expected by the pilot and he blacks out and slumps down (apparently why you can't see the pilot in the crash video).
Apparently the thing clocked around 500mph in qualifying so it would all happen pretty quickly.
In an eerie twist of fate, an Australian was apparently asked to fly the thing at Reno but then the owner decided to fly it himself.