Some of the separation can be attributed to exposure to temperature variations. As each material (metal, plastic a, plastic b-coating, etc.) expands and contracts at differing rates, separation at mostly the edges where stresses are greatest begins to show up as a "feathering discolorization"
Don't store cds or dvds near heat or near cold sources as this will reduce life of the data. Constant, cool temperatures are the rule.
Cheers, Hank