The two most common reasons for a parrot to pluck are stress and diet.
If the bird is stressed out, abused, ignored, or treated in a manner the parrot finds stressful, they will pluck because they need to focus their stress. If people or parrots can't release their angst against someone or something else, they will direct it inward. This is why people, most notably teens, cut themselves.
The other reason is diet. If parrots don't receive proper nutrition, they become stressed and ill. Again they will pluck in an effort to reduce the side effects of malnourishment such as itchy skin, skin disorders and the like.
Parrots who pluck must be put on a healthy diet, given skin soothing baths, reassured they are loved, treated with love and compassion, praised when they don't pluck; and, receive total support.
However, to turn a plucker around is a very rewarding experience. You are taking a bird that was in emotionally and/or physically very poor shape; and, giving them a happy, healthy life.