The tailwheel pops out due to the high G load. Looking at another forum on Pprune a similar incident occurred a few years ago when another P51 lost it's trim tab. The pitch up was around 10G and the pilot got jammed between the control stick and the sidewall of the cockpit. Only difference between this incident and the accident is that this aircraft continued straight up to around 10,000ft where the pilot was able to release himself from the jam he was in and land the aircraft.
The smoke is steam from the modified cooling system in the aircraft. They are speculating that due to the large amount of coolant carried onboard it may have inhibited impact fire.