I don't know what causes it.....You could always call your vet and ask what they think. Someone else on the forum also might know about it.
In the summer I brush my horses hours at a time sometimes 2+ times a day. It's a bit different this time of the year though, just make sure you watch them because when they start loosing this time of year they can get real cold. I've been brushing my horses 2 times a day but I haven't used my shedding block or curry comb very much yet.