First, you can't have "zero pressure" unless you are in a vacuum.
Anyway, when the flow separates, you get a separation bubble which is a local area of the flow where the boundary layer has detached and formed a circulating bubble between it and the surface. In this bubble, velocities are typically much lower than underneath. When you slow a fluid down, the static pressure rises, so now you have a higher pressure above the airfoil than below in certain points. That can lead to a loss of lift, and in some situations, even the opposite effect.