Kem Poker Cards

I have this game and Kem playing cards I got for about a month now and have used it in about 10 games. Recently, when I opened the case for using it a little swollen and has cracks in it as broken glass spider webbed. Any info about how it happened and what to do to avoid it would be useful.

It seems that the humidity was in the case and you let the cards heat or direct sunlight.