someone with more experience will chime in soon i bet.. maybe they will tell you to look for mites or if its from sunburn? do your pigs have a wallow spot? that is very important for thier skin.
I don't know what type of pigs you have, but I rub my pot belly pig with Avon's skin-so-soft bath oil and lotion to keep her skin moist. It has the added benefit of detouring mosquitos as well!