I found this reasonable explanation online:

I've seen this phenomenon before. When my dear Clyde first began to lose his mask I took him to the vet. Medical science was powerless to explain why his mask was rapidly disappearing.

Then a friend suggested that I take him to an animal behavioral psychologist and we went. This doctor has given me the most unexpected news; Clyde is apparently self actualizing!

I didn't really know what that meant at first either. Over the last two years Clyde and I have been through a real journey. At first he bit me - he bit me a lot. Then his restless ways resulted in the destruction of most of my furniture and all of the carpets. Eventually Clyde and I had it out. I remember the big breakthrough as if it were yesterday. I sat him down and explained to him that this couldn't go on. He sprayed me. Undeterred, I emptied my heart to him. I told him of the heartbreak his destructive ways have caused me. I explained that I would do whatever it takes to change if only he would be a happier raccoon.

That day I saw a look in his eyes that was undeniably understanding, we had had our heart to heart.

It turns out that Clyde has finally become the raccoon he's always wanted to be. I set up a garbage can in the living room that he's allowed to rummage through (turns out every raccoon needs to do this, it's instinctual). I make sure that at dinnertime I give Clyde his food first, before I eat, and I never ever go near his favorite chair. Since then, he and I have seen eye to eye, and the biting has dissipated into nothing more than the occasional hiss.

Ever since his transformation, his mask has gotten progressively lighter. Today it's at the point where one can barely see it at all. The behavioral psychologist explained it like this, Clyde is finally comfortable as what he really is, and no longer needs to hide his true self.