As changogab said this seems to be very common in captive pairs, yet it is not seen in the wild. To state the obvious: there is something different about the captive environment versus the wild state that is contributing to this behavior. To state the challenging: find this difference and the problem will be solved - okay habits are hard to break, so maybe not solved, but minimized.
The most drastic difference between a captive and wild environment is likely the food source. I don't know of any breeders who make their birds forage for their food, but if they did it would be interesting to see if the incidence of this plucking behavior was reduced.