When you push air out of the sucker, the air pressure inside the
sucker drops.
So what happens is that there is a net force, the force acts on
the sucker from the outside, because the air pressure outside is
greater than the air pressure inside.
Thus, it is difficult to pull the sucker directly from the
window (perpendicular pulling). You will find, however, that it is
easier to slide the sucker along the window.
