Hi R. -
This type of behavior is rarely 'just because' and is usually symptomatic of an underlying issue and/or part of larger disorder - and I'm sure you've already heard that. Usually there is depression, anxiety, or obsessive-compulsive isses as well.
First off, find a very, very, very good therapist to treat your daughter. If you need help locating one, please let me know. If you are in the Bay Area, I have an EXCELLENT therapist who only works with teenaged girls -- many of her clients are 'cutters' and she has great success with them.
Second, consider meds. Again, you need to find a very good psychiatrist to deal with this -- and only after she has been seen by a good therapist who can accurately identify the underlying issues (anxiety and/or depression, etc.). Once you have a handle on issues, you can treat them with psychopharms.
In the meantime, the most important thing to do is LOVE HER. Try not to get angry with her, show disappointment or frustration -- she will feel that and it will only make her more anxious and more likely to continue the behavior.
Remember, as 'dysfunctional' as this behavior is, it is her way of COPING with stress -- it is the 'best thing' she can do to take care of herself. The best way to help her is to help her find an alternative way of 'taking care of herself' when she is stressed.
If you have questions, just contact me. HUGS!!
J.