You have to change the direction of airflow to create a high pressure zone, and a corresponding low pressure zone. So say you have a vortex travelling down the car, and a high pressure zone somewhere along the length of that vortex. That high pressure zone will make the vortex go around it, when that high pressure zone is disturbed(cross or tail wind for example), then the vortex will no longer be steered by that high pressure zone disturbing the vortex which is properly aimed when the high pressure zones are in place.
When you have water going down the drain and it forms a vortex, you can see the vortex can be disturbed, and water flow down the drain slows. Same with the vortecies travelling down the sides of the car.