I think it could be a couple of things. My cats used to get these in GA when we had fleas in the house. My remaining cat still gets them, but they're different now. He pulls his hair out. I laughed when the vet told me this was the likely reason. But, then I started paying attention - sure enough, he pulls his hair out (with his teeth).
Maybe also allergies, a rash, etc.
And, oh yes, a cat could scratch hard enough to bleed......