I haven't read every post but I've seen some comments about friction with the road causing heat. Depending on how you read it, that's not entirely true. It is caused by friction, but mostly the friction brought about by the sidewall flexing. The faster the tyre rotates, the faster the sidewall has to flex.
That could have been caused by under inflation at any stage in the tyre's life, or just a defect. The clean 'break' isn't anything to be surprised about IMO. The instant the carcass collapses, the tear will follow around.