I believe Rob and Nick are correct about this - the epoxy was probably not at full cure and has disbonded from the glass fibers. Localization of the whitening along glue lines indicates more movement (strain) there.

Just because epoxy is fully hard to human senses, doesn't mean it is fully cured. If fully cured epoxy is heated sufficiently, it will reach a glass transition temperature where it changes from a hard glassy solid to a soft rubbery solid. For a given formulation, the glass transition temperature of partially cured epoxy is lower than that of fully cured epoxy. Absorbed moisture will lower the glass transition temperature, but the cure state has a much larger affect than moisture.

Sam