Since the decals in question looked fine when originally applied to the model, but only yellowed after, my guess is the glue used in their manufacture (the adhesive layer between the backing sheet and the decal film itself) is of pretty poor quality or has impurities in it, and once activated by water, a slow chemical breakdown began. Too bad, because there's not a whole lot you can do now to fix them. Maybe a dose of direct sun would help, but that would also tend to fade the colored areas, too.