She was the one hitting on him. So yes, she was the aggressor.
People who keep saying this have NO idea what happened before the traffic stop. I have seen perfectly sane women be goaded and goaded until they finally blow up. A couple very close to me, the man said "she just loses it like batshiite crazy." And then I once saw how he completely drove her to that state.
Gabby's best friend said Brian was very controlling and jealous and had "episodes". It's my guess he had an "episode" that day and she fought back.