I'm lost on why some carpets bubble up after a few months

I mean, I followed all the steps for stretching and securing, but I've had two jobs where the carpet started to ripple. Idk if it's the humidity or maybe the padding I used. Maybe it's the way the subfloor was prepped, but I checked it and it seemed fine. It's just frustrating when you think you did everything right, and then you get a call back.