my theory is that the wipers are controlled by the camera in the rear view mirror assembly. So, when you jump in the car after its been raining, the wipers go on max speed. After a few swipes, the window is good. But the camera above the wiped area still shows a bunch of water droplets on the windscreen, so the wipers keep moving @ max. Once you start driving, the wind and aero dynamics blow a lot of water off the area under the camera, and then the wipers finally slow down.
That's my story and i'm sticking to it until some Rain Ai comes out!