That seems incredibly fast for plastic to degrade like that. I wonder if you could have something generating ozone in there.
My first thought exactly. I know someone who inadvertently melted a ton of stuff with overuse of an ozone generator, similar kind of effect
I'm not sure either. It's not an air tight bomb shelter and it's used like an average storeroom, storing stuff like winter jackets, suitcases and paint. I do use small amounts of Calcium Chloride based dessicants to keep the room dry.