As the wind curls downward off the top of the car it drives it onto the flat rear of the SUV/van.
On a car it keeps going past the rear window to the trunk and off the very rear of the car.
So when it is raining or snowing or you are on a dirt road, it all ends up on the rear window in a way that does not happen with a sedan. Used to happen with our old station wagon in the sixties and seventies but I think Ford had not yet figured out how to have a rear wiper.