If he just wanted to escalate the whole thing, he could have simply arrested her on the spot the moment she violated the law by failure to signal.
I don't think anyone has argued that he wanted to escalate the whole thing at the outset. From what I saw, the whole interaction started out as a pretty routine traffic stop. The officer was professional, and even courteous...to a point. But then it took a turn, and his ego got tweaked, and it seems like he lost his head and failed to deescalate.
She could have just complied with what he asked and received her warning and been on her way.
She could have. But was she legally obliged to do so? Rudeness isn't a crime.