My observation of preening within a flock of five Goffins suggests some of the propensity is personality. They all have nearly identical environments, same food, level of attention, in excellent health.
My sole overpreener is rather neurotic and high strung. Conjecture whether there is a parallel.
There may be parallels between overpreening and plucking. No guarantee the former leads to the latter. FYI, an excellent thread concerning plucking: http://www.parrotforums.com/behavioral/52217-plucking-search-answers.html