First, snowcap is an appaloosa term referring to a white blanket over their hips with no spots.
Second, what you are referring to is called frosting. I am not sure if anything actually causes it. Some horses just have it and some don't. It is also common on duns and occasionally grullos. My buckskin only has frosting in the spring when she's shedding her winter coat, then it goes away with her summer coat.
Is that your horse? That looks more like tobiano turning the mane white than normal frosting, which is just lightened guard hairs on the mane and tail, like this: http://www.theransomranch.com/images/sold/richard.jpg