I'd go with heat too. Like if it spent a bunch of time on a flea market dealers table, and florida gets pretty hot.
There's different types of Poly, and I don't have a Star bag. But I will say that regular penny sleeves will become too brittle to touch after a few years of sun exposure. They'll actually protect the card from fading, which is sort of cool.
I'm not sure how sealed the bags are, but if they're totally sealed, then whatever air was in there when they were packed is what will still be there years later. If they were packed humid (or even realtivly dry) the cards have a water content. Heat makes the water evaporate, then the AC in the car on the way home condenses it. I had that happen with a cheap small set of cards I bought from a guy who did his own bag sealing. bought on a day that was around 100, got it home and there was water in the bag. It was only a $1 set of junk, so it wasn't that annoying.
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