With your experience with warbirds, do you think that the tailwheel may have extended itself because of the high Gs, or possibly some stress on some component in the rear of the fuselage caused some damage, such as dropping the wheel and maybe something snaring the ‘upper’ elevator cable, putting additional aft pull on it.
Losing the trim tab would seem to be cause for trouble at that speed, but why it pulled up seems counter to the function it was performing.