Thanks all! I was leaning towards environmental damage initially, but the reason I had doubted that was because the color separation continues seamlessly along the rim and around the edge of the coin, and the obverse has equally little wear over the shiny and dark areas. But the car cup holder damage sounds like it would make sense if the coin was obverse side up and some type of staining liquid dispersed over the obverse, down over the edge, and pooled down into the reverse.