One other thing - if the insulation did damage the tiles, it seems likely so some of the total damage was done on the way up. 80 seconds after liftoff isn't much after Max Q - the point of highest aerodynamic stress.

There's even the possibility that the tiles might've been further weakened in orbit due to thermal cycling. Any voids in the adhesive might've grown larger as the tiles expanded and contracted during the 90 minute day/night cycles (over 250 cycles during a 16 day mission). If, as the California observer claims, pieces started coming off so early in the reentry, this might help explain why.