What really causes a low-pressure gradient over the top of an aerofoil?
This is my understanding: A combination of the shape, angle of attack and movement of the wing creates high pressure on the front side of the wing and low pressure on the back side. By front and back I mean the part that is facing the oncoming air and the part that cannot "see" the oncoming air. 

The low pressure is there because of this. This is the pressure difference which causes lift. Some lift also comes from the downdraft caused by air moving downwards at the trailing edge of the wing due to the angle of attack.

This article has a video that shows airflow over a wing.
http://www.cam.ac.uk/research/news/how-wings-really-work