Most likely this is a silly question but in the northwest it does rain a bit and when the doors of the X5 are opened, especially the rear, water on the top of the door frames always flys it way onto top side of the seats.

My rear seat passengers make a comment swipe it with their hands or just sit on it. I figure there is a need to keep the car slippery in the air and may be the reason but is this common to everyone?