Bravo Sierra. Judging by the plume, I'd guess there was a cracked casing in one engine resulting in reduced thrust for that engine. With asymmetric thrust being produced by the cluster, the inertial guidance tried but failed to maintain the programmed trajectory by rolling the booster. Looks like the gyros went into gimble lock, and the whole stack tumbled and broke up under dynamic pressures prior to impact.
At least, that's my take based on the video.
Now I would believe all engines shut down about a second before impact due to range safety interlocks. Up to that point, however, engines were running.
Helluva firework...