I currently have a yellow crowned amazon but have had birds all of my life. Fortunately, we haven't experienced feather-pluckers. I feed a pelleted diet with daily fresh food and there is a weekly or twice weekly shower to reduce dander etc. I think aside from disease or mites, it could be dietary but most likely behavioral.
I did find this link which might be helpful written by an avian vet.
The Basics of feather picking