She could have tripped and simply put her hand out to catch herself on the back of the car.
Or even just anxious, walking fast, and like you said, caught herself on the car. They didn’t have their cameras on so we will never know.
That’s how people outside a car get the attention of those inside.
Little woman slapping a patrol car is deadly force, of course, requiring a subdued corpse
Or, perhaps, the “slap” simply didn’t happen. Put the Jihadi murderer and his partner, who’s covering for him, under a polygraph and watch the needles wildly jump!