Disk rot usually happens when air gets in contact with the reflective coating and oxidises it. With CD’s, it’s actually the top side you need to be worried about, as it’s right there under a thin lacquer coating. Any ding to that can expose the layer or just literally chip off a chunk of data.

At least on DVD’s it’s sandwiched inside the disk, so usually the only reason is a manufacturing error, and not really something the user can cause.