Why does my female tiel pluck her devoted male partners head?

Twinky (female) plucks Rickys (male) head every time they preen eachother. Ricky usually preens first then when its Twinkys turn to preen and he least expects it RIP-SQUAWK she pulls another feather from his nearly bald head. He is so loyal to her he goes back for more a little while later. Same routine over and over- I feel so bad for the little guy. Anny suggestions?