I wonder if he could have been bitten by a venomous insect or spider and the tissue necrosed? Years ago, I had a dog that took a brown recluse bite to the face. That got pretty ugly before it finally healed. (and of course, it was my show dog, not one of the working farm dogs)
We'll probably never know for sure what happened, but we can have all kinds of fun thinking up theories. I'm just glad he's ok and he will be able to grow enough hair that it won't show. And, most importantly, that you weren't the one brushing him when it fell off. lol
Give the big guy a huge hug for me.