I think this may be a result of the sleeves I put them in but when I store holos in the transparent penny sleeves, it seems the cards begin to curve or bend. The front part curves upwards creating a card hill. This concerns me to store anything in penny sleeves anymore. Anyone else have this happen or give me any tips on how to avoid this?
That is a natural occurrence with almost all holographic cards. If the bend is slight I wouldn’t worry to much as it won’t effect condition. However if it is a bit more pronounced or you just want to make it absolutely flat, store them in a top loader or in a binder with a little pressure.
Any idea why this does not occur until they are put into sleeves? I had cards sitting out for a few weeks. Finally decide to put them in a sleeve, a day later they are curved already.
I have never noticed that. Whenever I had holo cards they curled both with and without penny sleeves. Anyway, it is reversible as I sated above 
but I will give it a try when I have a chance and see if I notice a difference.
Ah yes humidity and air are factors that definitely could make an impact. Thanks.