The heat tiles were damaged by something.
From the telemetry, it appears that extreme heat forced its way into the orbiter and consumed it from within. Had but a few of the tiles been knocked loose, the stress of reentry would've removed them and the heat of friction burned into the orbiter like a blowtorch. After that, aerodynamic forces would've taken over and finished the job. T
This was a freak accident that very little could've been done about. Had they known of the damage in orbit, then what? There was no way for them to dock at the ISS, and how long would it have taken to launch a rescue mission?