Okay, sounds like she needs a time-out. If you have a dog crate or a pen or some place to keep her so she is separate from the rest and then observe the behavior of the remaining birds. Pullets have to establish a 'pecking order' which can look very aggressive, but you said you've had chickens for years, so you know what that looks like. It might just be that the Brahma is a bully. If you separate her and the rest of the girls calm down, you'll know who the instigator is. By separating her out for a time, and then reintroducing her to the flock, you might get her to settle down and behave herself.
