I am having the same issue with one of my hens. She went bald practically over night. She and the other EE are my two top hens that are always switching who is first and who is second. I treated for lice and mites not too long ago and checked the otehr night to make sure this isn't a resurgence. It isn't so far as I can tell. It seems a lot of people have this happening but no one seems to have an explanation. Some say its because its the omega hen in the flock. Others says its because shes the alpha, some say she's laying, some say she isn't.
Whats particularly intresting is that the former head of the flock(which I culled a few weeks ago) had a bald neck like this too and I thought it was just because she was old. Then my 1 1/2yo EE starts developing the same feather-loss and now I'm positively stumped. I know she is laying because I only have two EEs, and I know she's not being bullied. Theres no roo in my flock who is roughing her up so I'm really at a loss.