from what I gather (and saw a snapshot of one of the other failures of the same booster), the thrust (either one of the vernier engines or some other reason) gets asymetrical, and the rocket actually starts to sprial, and of course, the vernier on the other side does try to compensate - so you have 2-3 engines seperated by a few feet going around in a sprial (from the small snapshot I saw, it looked like it went inside - OUT, and then was destroyed - if they were late on the range safety package, or there was none, I can see this happening