The process follows the Hooke's law, hence if you press the rubber suction cup against the wall, it's shape changes and it forms a pocket of air that occupies a volume that is smaller than the original volume of undeformed rubber cup.
Since the rubber is a polymer that has the property to return to it's initial shape, after the stress applied on it is stopped, then, the rubber suction cup will be regain it's original shape, after the pushing action stops. Hence, the pocket of air increases it's shape, but the volume of air remains unchanged. In the present condition, the air pressure inside the rubber suction cup is smaller than the air pressure outside, so this outside pressure pushes the rubber suction cup against the wall. It also occurs a friction force, proportional to the force exerted by the outside air pressure. This friction force maintains the rubber suction cup to it's place.
thxs a lot for your ans