Good analysis possibility. Having watched the video a number of times, there is a brief change in character of the thrust (more opaque, brighter), followed by a small explosion at the base of the rocket, then loss of thrust, descent and large explosion upon impact. The problem sure seems to be at the base of the first stage.
I wonder if it might have something to do with the delayed launch. The oxygen is “supercooled” and plays a role in cooling the engine. To me it raises the question of whether the refueling created some kind of condition different than the routine launch sequence.