Oxiser tank to rocket motor connection failure?
Trying to get something from the pictures.
From the NTSB B-Roll video 0:18 top of tank, 0:34 bottom of tank/interface (though hard to see) and 2:47 rocket motor(?) which appears intact at least the front end, one might think of a connection failure between the tank and motor, rather than an "explosion". Or the invisible motor bottom end burried in the sand may hide a failure that provoked the tank-motor separation. In both cases an immediate vehicle break-up would be a likely next effect.
The motor will have been subjected to ground tests multiple times, however itīs behaviour integrated in the vehicle in flight conditions cannot be investigated beforehand other than by modelling, and unexpected effects may be revealed in actual flight. Propulsion dynamics coupled with aero-elasticity? One can only guess at this stage.
Perhaps someone can fill-in at some point about state of tank-bottom and motor-bottom/throat/nozzle?
Just my 2 pence.