The final image of the Mustang about to impact the ground at high speed with very large nose-down pitch angle is terrible to see.  But then I noticed the elevator – there seemed to be no upward-deflection that one would expect from a large back-stick input from the pilot to pull out of the dive.

http://www.urbanchristiannews.com/ucn/2011/09/reno-air-races-i-was-preparing-to-die-crash-witness-says.html

This seemed odd.  Either the pilot was incapacitated (unlikely, as he had shortly before issued a mayday radio call), or the elevator was inoperable, or jammed.

Then I came across this picture of the trim tab breaking loose.  This is an amazing image, and points us in the direction of mechanical failure during manoeuvring, which resulted in the severe pitch-up and stall.

http://www.garagejournal.com/forum/showthread.php?p=1754475