“What causes adolescent aggression?”

There is no single answer to adolescent aggression or treatment for aggression. The key is to communicate with your child in a non-confrontational and non-judgmental manner to find out what is occurring in their mind. Your child may be struggling in school, being bullied, coping with depression, or even being abused by someone in the community. When your child becomes aggressive, try to ask what they need instead of scolding them. Once they feel that you do not understand, they are much less likely to open up and tell you the truth. Try to help your child co-regulate and continue to keep open communication. If your child is not willing to talk to you, bring them to a trained professional, such as a therapist or ask the school counselor to check in on them.

Great question. Some times aggression is actually depression in kids, depending on the age. Maybe start off with therapy. It also could be other issues as well such as medical and or drug use. Have her see her pediatrician for an evaluation and lab work up. And continue with therapy.
Hope that helps as first steps