[QUOTE=in2it;2598373]The pattern of damage rather suggests that the polycarbonate disc, to which the label has been attached, is porous, and, over time, an organic part of the adhesive has migrated down to the dye.[/QUOTE]
That is exactly correct.
Discs are also vulnerable to hydrogen sulphide which is a naturally occurring air pollutant and a combination of oxygen and moisture which migrate through the polycarbonate.
These are the primary causes of ‘DVD rot’ which is damage/degradation of the metallic reflective layer.
BTW labelling discs can also have an adverse effect on your hardware as it unbalances the discs and puts greater strain on the drive components.