just to make the point that entry height is somewhat unimportant.
It is unimportant (for powered aircraft) as long as nothing goes wrong in the manoeuvre. Clearly something went wrong in this particular manoeuvre. We don't know whether it was pilot error, or temporary pilot incapacitation, or a technical problem, or something else. Whatever the case, if he had started the manoeuvre even a few dozen feet higher, the incident would not have happened.
I suspect that the exact wording of the display authorisation and the aircraft parameters at the start of the manouevre are going to get looked at in some detail.