Protein is one possible reason and bordem is another. Some just get this as habitual (I call it addicted in some bad cases), though it'll most likely start with one of those first two reasons, or as a learned behavior. I suggest dealing with this as early on as possible. If it's just her, separate her long enough to break the habit (month at least) before she teaches it to others. (Separating her may also reset the pecking order as well, which in some cases, that will be good.) Add more protein to your flock's diet (if possible) and give them more entertainment and/or space (adding leaves are great for entertaining, along with added perches! ).