This may be a silly question, but when it's said that these are not in a "stable orbit", what is the mechanism of decay? I'd imagine if they were in an elliptic

This may be a silly question, but when it's said that these are not in a "stable orbit", what is the mechanism of decay? I'd imagine if they were in an elliptical orbit, they'd simply continue that way forever in a "perfect" system. I'd imagine tidal forces would be too weak and the satellites too small for them to have any effect. Are they passing through the atmosphere and experiencing atmospheric drag?