It's a fair question, and I don't have a good answer. My best guess is the effects of heat while the discs were in moving boxes for several cross-country and overseas moves. Some of our discs looked like they were delaminated.

writable DVDs degrade, but a typical DVD won't on its own for a long time in reasonable conditions. I suspect that over the years those library disks have been scratched to hell several times over and ave had to be buffed down to the point where it's causing problems even though they now look clean.