It is the plastic aging.
There was a lot of concern some time ago about plasticizers in holders migrating to cards--some collectors use polypropylene sleeves inside their top loaders or even use inert mylar holders as a result of those concerns. FWIW, I've bought cards from some old time dealers/collectors in holders that were piss-yellow with age and I've never seen a hint of damage to the cards in them.
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