A B737 PARKED AT A GATE ROLLED A FEW FEET WHEN THE GROUND CREW FAILED TO CHOCK THE AIRCRAFT PROPERLY.

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WE ARRIVED AT THE GATE WHERE A SINGLE MARSHALLER PARKED THE AIRPLANE. AFTER WE STOPPED; THE MARSHALLER CHKED GND PWR/AIR CONDITIONER AND FOUND IT TO BE INOP. THE ON GND CALL INDICATED THAT GND SVCS WEREN'T AVAILABLE; SO THE APU WAS STARTED. I INDICATED THAT BOTH ENGS WERE SHUT DOWN AND THE MARSHALLER THEN INDICATED THAT THE CHOCKS WERE INSTALLED. WE WERE EARLY TO THE GATE AND THE NEXT CREW WAS COMING FROM ANOTHER GATE; SO I RELEASED THE PARKING BRAKE TO HELP COOL THE BRAKES. APPROX 5-10 MINS LATER AFTER ALL THE PAX HAD DEPLANED AND WE WERE GREETING THE NEXT FLT CREW MEMBERS THE PLANE WAS PUSHED BACKWARDS 2-3 FEET. I OBSERVED THE SAME MARSHALLER (ALONE) STANDING IN FRONT OF THE TUG TRYING TO CONNECT THE TOWBAR. IT APPEARED THAT HE HAD POSITIONED THE TUG BY THE TOWBAR AND THEN DISMOUNTED THE TUG TO CONNECT THE BAR. THE TUG HAD SURGED FORWARD PUSHING THE PLANE AFT. INSPECTION OF THE AIRPLANE INDICATED NO DAMAGE AND NO PAX WERE HURT BY THE SUDDEN MOVEMENT OF THE AIRPLANE. SUPPLEMENTAL INFO FROM ACN 796367: FROM WHAT WE COULD DETERMINE; WHAT HAPPENED WAS THAT THE GND CREW WAS HOOKING UP THE TOWBAR TO THE TUG AND IN THE PROCESS OF DOING THAT; PRESSURE WAS APPLIED TO THE ACFT AND IT BEGAN TO ROLL BACK. THIS SHOULD NOT HAVE HAPPENED IF THE ACFT HAD BEEN PROPERLY CHOCKED. ENSURE GND CREWS UNDERSTAND THE IMPORTANCE OF BOTH CHOCKS BEING IN PLACE WHEN THE ACFT IS CHOCKED AND THAT THEY CONFIRM THE CHOCKS ARE IN PLACE.