Wavey sleeves

!PROBLEM SOLVED - ITS MOST LIKELY HUMIDITY!
The sleeves of one binder of mine got wavey, only the backside of the sleeve. No other sleeved cards were affected.
My question now, how can this happen? Is connected to humidity or can there be a way its connected to binder and sleeves, since its only the backside of the sleeve (the side that shows to the binder pages)?
The maximal humidity can reach 63% here. We never struggled with mould or wet.
Thanks for any information and help

This happened to the sleeve of only one of my binder cards. I think it’s because the sleeve wasn’t completely flattened when inserting, allowing small pockets of air to remain stuck between the sleeve and card. Then after long periods of compression, the sleeve will gradually contour to the shape you see now

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It did happen to all cards of the binder tho. But only the backside, the side showing to the binder page, how I said. I cant really explain it to me.

What kind of binder? What kind of sleeve? Is the sleeve opening up or down?

Sleeves are usually not very elastic (they don’t revert to their original shape when stretched.) As such, they wrinkle and stretch easily. If they were placed into pockets too small, this explains why they’re wrinkled.