Here is Millie the cat, looking quite normal.

Change the angle, though, and...

Aaah! Baldy-cat!
(Those wrinkles are normal, by the way. I think every housecat has them when its ears are pointing forward, but they're only noticeable when there's no fur in the way. This confused me the first time I saw a Devon Rex; I thought he'd been shorn with a very coarse clipper, or something. Then I noticed all of the other wrinkles.)
How this happens is, Millie thwacks another cat, usually Mickey, on the head.
Or Mickey thwacks her.
They seem to take turns starting it.
Then they get into it a bit, usually in the completely-silent-except-for-the-thump-of-furry-bodies-on-the-floor way that means that nobody's taking it too seriously.
And Millie, because she don't take no crap, sticks with the wrestling and thwacking until Mickey gives up and wanders off.
The result, now and then, is a small claw injury on Millie's head.
(Injuries on the head: Brave cat. Injuries on the bottom: Cowardly cat.)
This little scratch would heal perfectly normally, if left to itself. Cats can't clean the tops of their heads very well, but you don't often see an abscess developing elsewhere than around the ears either, even in cats that get in fights every night.
Joey, however, believes it is his duty to keep Millie's head very very clean. Or he just likes the taste of scab, or something. Anyway, he comes along and grooms the living bejaysus out of Millie's head, day after day after day. And his rough little tongue expands the injured area until Millie looks as if she head-butted a disc sander.
This has happened twice now.
After a week or two of the feline horror show - which does not appear to actually bother Millie at all unless you touch the ouchy bits - it all heals over and she's just left with a bald spot, which gradually fills back in with fur.
(There could be a vicious circle thing here, too, in which Millie's ouchy head causes her to be more aggressive when Mickey idly thwacks her on the head in passing, thus encouraging more injuries.)
Oh, and for the benefit of the search engines: Bald pussy.
Thank you.