Refer to figures.
Ground effect occurs because the surface alters the airflow pattern around the wings. Primarily, the surface restricts the formation of the wingtip vortices.

This results in a reduction in the amount of induced downwash behind the wing and increases the wing’s effective angle of attack.

It also alters the pressure distribution around the wing and the amount of lift developed. This occurs because the change in effective angle of attack increases the wing’s coefficient of lift (C_L) and thus its lifting capability at any given angle of attack.

The magnitude of the wing tip vortices, and thus downwash, also determine the amount of induced drag produced by the wing. The closer the aircraft is to the surface, the greater the reduction in induced drag at any given angle of attack. (figure 2)

THE INFLUENCE OF GROUND EFFECT ON LANDING
When an aircraft enters ground effect during the landing phase of flight, the sudden increase in lift and reduction in drag cause it to experience a floating tendency. The aircraft also tends to pitch nose-down due to the reduction in download on the tailplane.