A B737 ROLLED BACK; ITS DOOR MAKING CONTACT WITH THE JETBRIDGE DURING BOARDING WHEN THE TUG'S BRAKES FAILED.

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ARRIVED AND PARKED AT GATE. RAMP CREW SIGNALED CHOCKS IN. ACFT BRAKES RELEASED AND THE PAX DEPLANED. THE CAPT AND FO MOVED THEIR PERSONAL GEAR TO THE JETWAY AWAITING NEW CREW FOR FLT DECK CREW CHANGE. THERE WERE 10 THROUGH PAX AND FLT ATTENDANTS ON BOARD. THE RAMP CREW ATTACHED TUG/TOW BAR TO ACFT AND REMOVED THE CHOCKS. WHILE THE CAPT AND FO WERE STANDING IN JETWAY; THE OPS AGENT BEGINS BOARDING PAX. AS THE FIRST PAX APCHS THE JETBRIDGE AREA; THE ACFT ABRUPTLY BEGINS ROLLING BACKWARDS. THE FO STOPS THE PAX AND HAD HER AND ALL THE OTHER PAX MOVE TO THE SIDE OF THE JETWAY. THE ACFT ENTRY DOOR STRUCK THE JETBRIDGE AND THE ACFT STOPPED ROLLING. THE ACFT WAS INACCESSIBLE FROM THE JETBRIDGE. THE CAPT WENT DOWN TO THE RAMP JUST AS THE RAMP CREW ARRIVED AT THE TUG. RAMP CREW INDICATED THAT THE PARKING BRAKE ON THE TUG HAD FAILED. RAMP CREW PULLED THE ACFT FORWARD. THE OPS AGENT CLRED ALL THE PAX FROM THE JETWAY; DEPLANED THE THROUGH PAX AND FLT ATTENDANTS. MECHS CAME TO INSPECT THE ACFT. IT WAS REMOVED FROM SVC. THE EVENT STEMMED FROM A FAILURE OF THE TUG BRAKES WITH NO CHOCKS INSTALLED ON THE ACFT TIRES. THERE WERE NO EVIDENT PERSONNEL INJURIES. LEAVE THE CHOCKS INSTALLED UNTIL JUST PRIOR TO PUSHING FOR ENG START. THIS WAS A GND EVENT AFTER THE FLT WAS CONCLUDED WITH A FLT DECK CREW CHANGE OCCURRING.