It might be from boots rubbing the hair away. You could get some fleece boots and see if that helps
I had a horse who used to go almost bald in the late spring/early summer. We medicated like crazy only to find out that it was caused by flies.
Sometimes horses get weird rainrot or something on their legs. My mare got this even though she hadn't been out in the rain much. I fixed it by mixing Iodine with water and lighting washing her legs with it on a towel. That's what my vet recommended to me.