Interesting, it's more complicated than I thought, definitely gonna read more about it, thank you!
There have been quite a number of studies that have shown a relationship between birds who were force weaned and not allowed to fledge properly, and plucking disorders...
Not allowing them to abundance wean triggers abandonment issues... In the wild, it means the parent is going to leave them to die! So, yeah, you might have some psychological stress/issues with that one, if you were a baby bird!
Fledging properly is the key to a young bird's development, coordination, physical strength, and confident. If not allowed to do this, he falls behind other birds. He's different. He can't do what they do. He's perpetually "the runt." And other birds pick up on the lack of confidence and pick on him as well...
So those are factors in the psychological development of plucking disorders.
Then when you add to that the fact that Toos and CAGS live in flocks of 100+ birds, where they have constant companionship, stimulation, and comfort... something we just can't give them in captivity...
Then throw in the human caused components of neglect, and isolation.
You have a plucker on your hands. Reversing that one is hard!
Physiological plucking is easy. Guess right. Change the diet. Figure out the allergy or toxin. Don't expose the bird to the trigger anymore.