When the aircraft lifts off, the landing gear squat switch tells the cabin pressure regulator that the aircraft is now airborne, which then causes the outflow valve(s) to "slam" shut, so the cabin pressure rises suddenly.  This momentary sudden change (rise) in pressure is cabin bump.  For simular reasons the pilot is supposed to select a cabin altitude of 1000 feet or so above the runway prior to landing.  This insures that the controler has gradually released all cabin pressure before the squat switch calls for outflow valves full open, wherein the passengers brains all go squirting out their ears.
Hope this is clear enough to help.