Both of my cats venture outside, but come inside to eat and such. One of them is an manx cat that has a very bad attitude problem. He will attack ANYTHING..humans, other cats, even really big dogs. Anyway, he has this thing about pulling out chunks of his hair when he gets mad. We have taken him to the vet to see if he had mites or something, but he was clean. The vet said that it was probably due to stress, kind of like those cases where people develop an obsession with pulling out their own hair when stressed. Maybe it's the same case with your cat? Boots (my cat with the attitude problem) didn't start doing this until a few years ago. He's probably 9 years old now. It's a stab in the dark, but I figured I would mention it anyway. Boots walks around with bald spots on a regular basis.
Good luck with your cats!