The reason...
450+mph.http://img851.imageshack.us/img851/2903/ypwe1.jpg
Trim tab fails.
High G nose up attitude because of broken trim tab.
Pilot blacks out, seat collapses, etc...
Pretty much spells it out.
This is not the only thing ive noted that has gone wrong, in a few other pics there is a burst of steam/smoke from behind where the radiator intake should be (has been removed for streamlining i guess) which can be seen in another pic posted here, and also in another picture the tail wheel has extended. To me it seems it could be that something has blown out, throwing bits that have damaged other parts of the a/c, such as the trim tab and tail wheel locking mechanism.