Wikipedia's explanation of the sequence of events:
The O-ring failure caused a breach in the SRB joint it sealed, allowing pressurized hot gas from within the solid rocket motor to reach the outside and impinge upon the adjacent SRB attachment hardware and external fuel tank. This led to the separation of the right-hand SRB's aft attachment and the structural failure of the external tank. Aerodynamic forces broke up the orbiter.
I understand that lots of debris was being created by the SRB destruction, and the destruction of the fuel tank intuitively seems fatal. But this text doesn't go in that direction. In spite of the various debris, it is stated that aerodynamic forces tore up the orbiter itself. Maybe it's a combination of the two, but it certainly implies that aerodynamic forces were sufficient to break up the orbiter.
But this Space Shuttle was climbing, not reentering. That should put it well below orbital velocity and (I would assume) well below the velocity that the reentry vehicle sees at that altitude. If it has a heat shield which can withstand the tremendous aerodynamics of reentry, then why was this point in flight obviously destructive when the attitude changed?