How old is your mare? Does/did she have a thick winter coat?
I have an older gelding, Sammy, with a thick coat that it coming out, literally, by the fistful (the yard is covered in little red clumps, as it my jacket, and everything else). It's strange compared to my younger gelding, Loki, who sheds rather normally and not in thicks bunches of fuzz.
Sometimes when a horses hair falls out in clumps, they are actually rubbing and scratching on things because they have lice. You can get de-louse powder at most feed stores. Also worming her will help. But you must at least use the powder...
Happend to my horse last year. The problem was she wasn't getting enough nutriance in her hay (we switched to difrent people this year) so her winter coat was that good of quilty so it was falling out before the new coat could grow, there fore the coat would fall out in clumps it was crazy. And when it did the new hair wasn't growin in all the way yet. So that might be the case.