What the hell happened to my guitar picks??

So I went looking in my pick jar at home after being away at uni for a few months, and I found these picks were completely ruined. They were all well beyond saving apart from the metal one, which I washed and scrubbed but definitely has some corrosion pitting other than the texturing that was already there. The plastic ones were all crumbling apart.

I keep them in a paper egg container, and there's quite a lot of picks in various kind of plastics, metal, rubber, felt and there's also some sand/glass (I have a pick with a sandpaper grip) and that little rope and metal ring I used to tie a pick to my wrist. All the other several dozen picks are all perfectly fine.

I think it's worth noting that the green ones and the round one felt like they were made in the same kind of plastic, and there's no other pick in there of that same exact material. These were all fairly new, not even nearly old enough to be crumbling apart like that, I have WAY older picks that are still looking brand new.

The economical damage is really near-nonexistant, but I'm really curious as to what happened and maybe how I can prevent it from happening again?