He only told her to get out of the car when she refused to put out her cigarette. Seems to me, and it apparently seemed to Sandra, too, that he had begun a process of hassling her trying to get her to give him an excuse for escalating things beyond a mere traffic stop. But she became outraged and ended up taking the bait.
That's how I interpreted the encounter as well. I think what probably happened was he thought she was acting like an uppity bitch, which he took great exception to, and instead of de-escalating the situation he inflamed it by giving her a "lawful order," probably secretly hoping that she wouldn't comply so he could teach her a lesson in humility.
It was a comedy of errors on the part of both of them. Both of them let their emotions dictate their actions. She should have remained calm and compliant, within reason, of course. Going off the rails like she did only exacerbated the situation. And he, well, he should have known better. His complete lack of tact and professionalism by demanding she do something that was in fact not a lawful order (extinguishing the cigarette) is what set the incident in to motion.
It just spiraled out of control after that.
"Reason obeys itself; and ignorance submits to whatever is dictated to it" -- Thomas Paine