Someone has suggested a heart attack and another offered passing out from G forces.
Neither is likely since (A) these guys are the most medically checked and approved breed on earth, (B) since he'd been racing for years, he'd been hitting those same G forces for a long, long, time without blacking out, and (C) the forces in a race such as this push back and down - into the seat - not the head slamming & canopy banging you'd see in aerobatics.
Although this 'plane had raced a lot of times before, I don't think it is far-fetched to consider the seat simply breaking away from it's base, or the base from the airframe, etc. Bolts can work loose or be left loose after maintenance, and welds can fail from long use or poor technique.
It is also likely that any bulkhead behind the pilot and any equipment originally instilled there was long gone to reduce weight. He could have gone a long way back in the fuselage and a long way away from the controls and then held there because the aircraft was accelerating and he was no longer attached.
Which leaves the very chilling thought that he was trying to climb back to regain control the entire time from climb to crash.
“A couple of things:
First; I’m a bit smug at the thought that I noted the absence of a pilot as soon as I saw that photo yesterday.”
We were already discussing this here last Saturday a few minutes after the new photos came out.