It's intriguing that this failure occurred at level-off. If the THS failed and slammed into the full UP position (nose-down trim) could the resulting force of it slamming against the stop cause sufficient loading for it break out of the tail, remembering that aerodynamic forces will also be grabbing the tail as it moves?
The first deviation in altitude was a sudden loss of 250ft followed by a climb, and however unreliable the subsequent barometric altitude figures were it looks like it dropped suddenly at TOC then climbed severely, which might support your theory?