The two threads about plucking recently were the problem with a cock bird plucking the babies and trying to kick them out of the nest so he could breed his hen again. He simply wants more sex and the only way to get that is with an empty nest box. The only solution to this problem is to remove the abusive cock and allow the hen to feed the babies till them fledge (leave the box freely) when the cock is to then take care of them, following them on their fledgling flights and feeding them when they need it. If the youngest baby is at least five weeks old, one can turn them out of the nest box onto the grate and remove the nest box to foil the cock. This sometimes works, especially if the older chick finds the open door of the cage and takes his first flight and turns daddy abuser into daddy caregiver through instinct.
Otherwise, plucking can be in response to a pain like an injured bone, a skin condition like dryness, etc, or idiopathic which is fancy Latin for "I don't know". If you have questions about plucking and pluckers you could look up some of the old threads in the bird or birds you are interested in section. KNOCK ON WOOD, I have had only one plucker and this was a hen who I feel was forcibly raped and abused by her brother cock. She was afraid of cock birds in general and lived in a cage completely populated by hens for her entire 13 year life. My vet and I felt her plucking was related to her abuse by her brother. I have never let a brother/sister pair close to sexual maturity alone in a cage again; always at least one other bird, and usually a pair, with them.