It still looks like dappling to me. I did a quick search on Google for "reverse dapples" and found these. If it was a disease, why would it be a beautiful dapple pattern?
Reverse dappling progression « The Equine Tapestry
Reverse dapple roaning « The Equine Tapestry
Body clipping « The Equine Tapestry
Lots of coat markings have texture to them. Dapples, appaloosa patterns and pinto patterns frequently have texture. I know because I owned a Paint and you would swear the hair of the colored patches were a different length than his white markings. I don't have any spectacularly dappled horses myself, but when dapples do pop up they look textured. So I don't know, I still think it's a seasonal coat change.
Here are some regular dapple grays. Doesn't the pattern look similar?
Braymere Custom Saddlery: That grey Percheron...