The Bose QuietComfort 15 headphones are the most comfortable headphones I’ve ever used (in addition to having amazing sound). One of the key elements of this comfort is the soft feel of the earcups, which use memory foam. After a while, the soft leather will start to crash as it is stretched and contracted, and the seams can split.
Even if you take good care of these headphones by putting them in the included case (which is very transportable), the thin leather that encases the foam padding will start to happen. It’s only a matter of time, and will eventually happen no matter how carefully you treat the headphones. How quickly they deteriorate depends on how frequently you use the headphones.
The padding on my Bose QC15 headphones began to crack and flake off, and it became so bad that the seam popped, exposing the memory foam. Thankfully you can repair the earcups.