When ever two planes (roof and side) of air intersect (at corners), there is a meeting of "pressure differences". One pressure plane is bound to be more than the other, and moves or gets sucked into the lesser one. This movement of converging/meeting air is constrained by the flow of air passing all around it, and tends to funnel or contain it. This funnel of air is called a
Vortex.
The example below is my own pick up truck, I suggest doing a Google image search for an image more matching your own vehicle.
S10 4x4 Pick Up pictures by kach22i - Photobucket
