That Doran made me wonder if it was a varnish thing, but I'd think the fronts would be obviously yellowed/browned as well if it was the varnish. I do have regular Topps cards that have varnish that yellows (and a few others too), but I'm not sure about the fleer Glossy, or why it would be just certain cards.
I can see it easily being environmental for a first or last card in a cardboard box, or the edges of a whole set in cardboard. But not so much for ones in a tin. Are they in cellophane in the tin? That should sort of protect them from most environmental damage.
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