When my friend's dog had fleas, he, as she put it "scratched his pants off" (ie all the hair off his butt) in a pretty short period. It does look pretty scary when your animal suddenly loses a bunch of hair! But it's probably bugs, and maybe - since you've removed some trees - some new ones they didn't really have to deal with previously have made a home. Ick.
I know the barn I go to has had a few horses that react just terribly to insect bites and lose hair just from the inflammation, nevermind the scratching, so much the fly spray can hardly keep up. Poor babes.