Horse Hair Loss?!?! Falling out in clumps, other horses might be showing signs of same.?

One of my horses is losing hair on his body. His mane and tail are intact, but along his face, behind his ears, along the belly, neck & hindquarters his hair is falling out. Of course it just turned spring and he is losing his coat. But everyday I go out to curry my horses. When I got to him, I could curry over the same spot over and over again until the spot was bald, the hair just kept coming. It came out so easily that all I had to do was run my hand over it a few times and the spot would be bare. It doesn't seem itchy and it's not scaly or anything, just smooth skin showing through. The spots are very splotchy, not in any pattern or shape so I guess we can cancel out ringworm... & I'm very familiar with rain rot and it's definitely not that.... and what concerns me the most is another of my horses is beginning to bald behind his ears and behind his front legs. I AM having the vet out next week, but I just need some answers before that, I'm really concerned.