Sometimes it is a bird that does not fit with the flock for whatever reason. If she goes back and things settle down, good. If not, let her go.
Once I had a full flock, and got a horrible predator that took out 3-4 head. Kind of terrible, but I noticed almost immediately that the remaining flock calmed down, even with the uproar of a predator attack, they were more peaceful. At that time, I just had too many birds for my set up. And this is a little off point of you post, but my point IS (good grief, this is getting worse not better) that once you experience the change to a peaceful flock, you realize how stressful it was. I think then you are more attuned to the tension in your flock.
A peaceful flock is a healthy flock, and you can't make a flock be peaceful without changing what is causing the stress.