It's called hydrolysis, it can happen to any kind of polyurethane based foam. Basically when moisture gets into the foam and causes the polymers to break down. Most likely to happen if they were stored in a humid and warm climate for long periods, or if they get wet and aren't properly aired out before storage.

If kept in a cool dry place the pu foam should be fine.

Tigers are fuckin indestructible. A few pairs that I wore rotating daily for like a decade and they were fine until I lost them in a fire.

I should get a new pair.