Same thing happening to both of my horses, as well as many other horses at the barn where I board. Started with their chests and moved on to their backs, flanks, and little patches here and there. Definitely think it's just the warm winter and early, unusually warm spring. Their bodies don't know what's going on, summer coats aren't ready yet and can't keep up with the shedding. Just for reference since people are saying it's happening to certain colors...mine are liver chestnut and bay, but it's happening to pretty much every color here. Paints with a lot of white and darker colors alike. I panicked a bit at first since my mare had terrible rain rot all over her body when I first got her 9 years ago now, and was scared it was a re-occurrence (she's never had it again in all this time), but all looks well. Skin underneath is smooth and healthy looking, might give them a medicated bath and/or use a spray on them anyway to be safe though.