I just reviewed the animation at the CAIB website, and I realized something...

When the first sensor glitches started the shuttle was facing into the Sun , and the bottom of the shuttle was exposed. Ordinarily ambient solar radiation has an intensity of ~1350 Watts per square meter. But when Columbia was re-entering the atmosphere the Sun was very low against the horizon, making the phenomenon of "total internal reflection" possible. Not only would Columbia experience ambient rays, but it would also experience reflected rays, with the sum total being a double dose of radiation of around ~ 2700 watts per square meter. That could get toasty.

I'm not saying that's the answer, but it's an idea anyway. It may have exacerbated pre-existing problems in the left wing.