I don't eat them, but I happen to know for a variety of reasons Skittles used to be made with gelatin, but a couple of years ago were 'reformulated' to no longer use gelatin.

I don't know the rules of being Kosher, but it's likely a result of that reformulation; maybe it is even just as simple as the fact that gelatin isn't there anymore. They are vegan now, as a result, when they were not previously.