I'm not sure either, but suppose a patch was saturated with condensation/rain water then frozen solid by the cyrogenic propelants. The expansion on freezing might start to jack it off the external tank, and the stresses of lift off break in free.
The wing could have been struck by a heavy chunk of foam reinforced ice...
Something that supposedly happened two missions back and hit one of the SRB's hard enough to put a good dent in it. After that mission the rumor is that some people were calling for a grounding of the shuttle fleet until the external tank "sloughing" problem could be addressed.