Very timely feature for me.
On Saturday, we rehomed our second sex-link due to feather plucking and eating. I can almost definitively say what caused it: stress, breed, and confinement.
Our Australorps and Golden Comets had been raised together as day old chicks, and lived peacefully for almost 6 months. I couldn't even tell you what the pecking order was. On December 2nd, one of the Australorp pullets decided she needed to guard the feeder from our remaining Golden Comet. Excessively. We immediately put out a secondary feeder, but the Comet resigned herself to foraging the Australorps' dropped scraps. We assumed it would sort itself out.
Two days later, the Comet was voraciously picking and eating feathers out everyone's behinds. She immediately got moved to the adjacent pen. We upped the protein and made sure she could eat in peace. Every evening, we would reintegrate and chaperone them before bed. The Comet would immediately go for their butt feathers. This continued over 5 days with no improvement. Rather than doing a long-term separation in the dead of winter, we rehomed her with a free-ranging acquaintance. If you've ever had a sex-link, you know that they're power layers with more critical nutrition needs. She spent probably 65% of her waking hours at the feeder. The food guarding Australorp interrupted that, and pecked at her head/comb/eye whenever she dared to try and feed herself. I'm sure this generated a lot of stress. As others have mentioned, feathers are very high protein. The Comet did what she needed to do to fuel those daily eggs.
Our first rehomed Comet pulled feathers in a different manner. Whenever the Australorps wanted to settle down during the day, she would come over and yank at whatever she could grab. It almost looked like a challenge, and it was CONSTANT. If the Australorps were up and about, she left them alone. And she never did it to her Comet sister. I don't know what to think about that situation, but she was rehomed to the same acquaintance. The behavior stopped immediately. In hindsight, we discovered she was a RIRxRIW cross, and likely had bullying in her blood. She never ate the feathers.
This was our experience. From reading through this thread, it's not uncommon to see sex-links (or production reds) as the offenders.