Ink is actually more stable on a glossy surface than an old cardboard one. On a glossy card, the ink rests on the surface, whereas with older cardboard cards, the ink actually bonds with the medium. This is what causes the ink to bleed over time (creating a slight haze around the signature) in somewhat rare occurrences on older Topps cards. The tradeoff is that signatures can be removed or "scratched" on a glossy card because no bond is made between the molecules of ink and the molecules of the item that was signed.
I have thousands of signed glossy cards that don't exhibit this fading so I don't think the gloss is a factor. Some type of chemical reaction may be the cause, but a more probable reason would be moisture. Direct contact with water would do this although I'd suspect the card itself would show more signs if the card had actually become wet. However this may have happened if the cards were kept in an area that had high humidity. Honestly, this looks exactly like fading from exposure to light, so I would question if the signatures hadn't been exposed to flourescent lights at some point, possibly before you acquired them?
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