Speculation:
The redundant set failed. What that means is the three computers that control the shuttle failed at the same time. That would cause major failures along the lines of the vechicle breaking up.
The other reason could be the APU's blew up. That's the amonia powered Aux Power Units. When they were started it appears the shuttle blew.
BTW Jay Barbaree at NBC is an old friend, he's the only talking head I would trust.