Rats will usually barber for one of several reasons, boredom, stress, and sometimes, they just do. Some people think it's hereditary. So, first thing you should check is her environment. Does she have enough to do? Enough space to play in? Is there something that might be bothering her (draft, loud noise, bright lights, etc.) If you've already checked all that stuff, and she keeps it up, she's probably just one of those rats that is obsessed by grooming. It's not harmful in any way, just makes you end up with patchwork rats.
