B757-200
Flight Crew Total 19000
Aircraft Equipment Problem Less Severe
Flight Crew Returned To Departure Airport; General Declared Emergency; General Maintenance Action
Aircraft
Normal takeoff; good plane no EICAS messages prior to departure.  On climb out passing 9;000 feet; we got a 'L AFT ENT DOOR' EICAS message along with the 'ENTRY DOORS' annunciator illuminated. Flight attendants by the left aft entry door reported hearing a pop from the door; [observing] the door handle moving up a few inches; and seeing light along the bottom edge of the door.  Cabin pressure was abnormal; showing a low differential and higher than average cabin pressure for around 10;000 feet.  We felt the pressure surge and higher cabin climb rate in our ears as well.  We leveled off the climb below 14;000 feet; declared an emergency; descended and returned uneventful landing.  Aircraft was 1;500 lbs over max landing weight; overweight landing was soft.  We stopped on high speed [taxiway] off runway; easy on brakes; and passengers remained seated.  ARFF met us; brakes were ok; and we taxied to gate.  At gate; left aft door was left alone; as it would require the Flight Attendant to push down on handle to unarm the door; and Maintenance met us immediately to inspect the door.We don't know why the door popped open during flight; but if there were item dropped along door edges; possibly causing the problem; maybe ground personnel servicing the doors need to be careful that things are not fouling the door.
B757-200 pilot reported a 'L AFT ENTRY DOOR' EICAS message on climb out through 9;000 feet. An emergency was declared and the flight returned to departure airport.