I don't really know what causes it, but not taking care of the disc doesn't seem to apply in my case. The disc has been in its case for the most part, stored away in a shelf, and none of the other discs exhibit the problem; basically the coating on the disc or the glue or something deteriorates and I'd assume would eventually affect the data.
I pull out the disc to watch it and the surface is all sticky and discolored and I can only get about twenty minutes in before it skips.
I cleaned it off with dish soap and water, which helped the disc to be read, but it still skipped at the end (and now there's no coating so I have to be real careful not to scratch the disc). Thankfully I was able to rip an ISO without error, but I don't have a dual-layered disc on hand to see whether it plays perfectly. (Usually if it rips without error it will, but I've had some exceptions where the data didn't copy right but said it was 100% complete.)
So yeah... The original disc is pretty much unwatchable since you can't see the end of the pilot without skipping, but hopefully the copy will work well and I can just watch that. I still asked CBS for a replacement, though.
I do wonder now, though, if having some of my DVDs stored where they are isn't such a good idea and if the conditions really did cause the rot. I can't see why it would be a problem, but they're basically stored in cupboards. Most of my DVDs are out in the open but I have no more shelf space, so I put 'em in the cupboards above my desk (spring-door cupboards, so they're shut pretty tight).