CATERING CREW APCHS AND OPENS THE REAR DOOR OF A B737-500; WHICH WAS IN THE PROCESS OF PUSHBACK AND PARTIALLY DEPLOYS THE DOOR SLIDE.

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AFTER THE 'BEFORE START' AND 'BEFORE PUSHBACK' CHKLISTS WERE COMPLETE AND THE JETBRIDGE PULLED; CLRNC WAS GIVEN BY THE PUSH CREW TO RELEASE THE BRAKES. BRAKES WERE RELEASED; AND THE FO WAS JUST ABOUT TO CALL THE RAMP FOR PUSH CLRNC WHEN THE PUSH CREW CALLED SAYING; 'THE CATERING TRUCK WAS BEHIND THE ACFT.' WITHIN A FEW SECONDS; A FLT ATTENDANT CALLED SAYING THAT SOMEONE HAD OPENED A REAR DOOR AND THE SLIDE HAD PARTIALLY DEPLOYED. I IMMEDIATELY SET THE BRAKES; AND COMMUNICATED WITH THE PUSH CREW AND THE FLT ATTENDANTS; AND DISCOVERED THAT THE CATERERS HAD DRIVEN UP TO THE ACFT WITHOUT ANY GUIDE MEN AND HAD OPENED DOOR 2L WHILE IT WAS ARMED AND WITHOUT CLRNC OR A 'THUMBS UP' FROM THE FLT ATTENDANTS. THE SIDE PACK CAME OUT OF THE CASE BUT DID NOT DEPLOY. AT NO TIME WAS ANY CREW MEMBER ABOARD MY ACFT MADE AWARE THAT CATERING WAS COMING TO THE REAR OF THE ACFT; AND THE PUSH CREW SAID THE SAME. OPS WAS CALLED AND MAINT BROUGHT A LIFT TO DOOR 2L; TRIED TO REPACK THE SLIDE; WAS UNABLE TO; AND ONE WAS BORROWED FROM ANOTHER ACFT. WITHIN A FEW MINS; MAINT HAD THE SLIDE PACK REPLACED; A NEW MAINT RELEASE WAS SENT; AND WE PUSHED UNEVENTFULLY. AFTER A LENGTHY ATC DELAY ON THE GND; WE CONTINUED UNEVENTFULLY. NO INJURIES OR OTHER DAMAGE WAS RPTED OTHER THAN THE SLIDE PACK. THE JETBRIDGE WAS NEVER RETURNED TO THE ACFT; AND THE ONLY DOOR OPENED WAS DOOR 2L.