What makes airplanes pitch up during landing?
The change in pitch during the landing is called the flare and it is controlled by the pilot (or autopilot for an autoland) using the elevators (i.e. pulling on the yoke). From the Boeing 737 NG FCTM (6.10 Landing):


  When the threshold passes under the airplane nose and out of sight, shift the visual
  sighting point to the far end of the runway. Shifting the visual sighting point assists
  in controlling the pitch attitude during the flare. Maintaining a constant airspeed
  and descent rate assists in determining the flare point. Initiate the flare when the
  main gear is approximately 20 feet above the runway by increasing pitch attitude
  approximately 2° - 3°. This slows the rate of descent.
  
  After the flare is initiated, smoothly retard the thrust levers to idle, and make small
  pitch attitude adjustments to maintain the desired descent rate to the runway.
  Ideally, main gear touchdown should occur simultaneously with thrust levers
  reaching idle. A smooth thrust reduction to idle also assists in controlling the
  natural nose-down pitch change associated with thrust reduction. Hold sufficient
  back pressure on the control column to keep the pitch attitude constant. A
  touchdown attitude as depicted in the figure below is normal with an airspeed of
  approximately VREF plus any gust correction.






The flaps are set much earlier during the approach phase. They are not changed any more during the landing phase.