AN ACR FLT IS TOWED BACK IN TO THE GATE WHEN THERE IS AN ECAM AND FLT ATTENDANT INDICATION OF SMOKE IN THE REAR OF THE ACFT DURING ENG START AND PUSHBACK AT CLT; NC.

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DURING PUSHBACK JUST AFTER INITIATING START OF #2 ENG; CALL FROM FLT ATTENDANT THAT THEY HEARD AN EXPLOSION IN THE AREA BEHIND THE AFT GALLEY. FEW SECONDS LATER; INDICATION IN THE COCKPIT FROM ECAM 'SMOKE.' FLT ATTENDANT IMMEDIATELY CALLED BACK AND RPTED SMOKE IN THE CABIN. CAPT ORDERED START DISCONTINUED; MECHS TO TOW US BACK TO GATE AND FOR ME TO DECLARE AN EMER; CALL FOR THE EQUIP AND JETWAY DRIVER. AS WE WERE BEING TOWED TO THE GATE ECAM CAME UP 'AFT LAVATORY FIRE.' PAX WERE EVACED AND SMOKE DIMINISHED. TOOK OVER 15 MINS FOR FIRE TRUCKS TO REACH ACFT. THERE WAS NO FIRE AND ORIGIN OF SMOKE HAS NOT BEEN DETERMINED YET. FA'S IN THE BACK RPTED HAVING METAL TASTES IN MOUTH AND THROAT FROM INGESTING THE SMOKE AND PROBS WITH SHORTNESS OF BREATH AT OCCURRENCE. EVERYONE EVACED SAFELY. RAMP TWR DOES NOT CALL FOR EQUIP DIRECTLY. CREWS NEED TO KNOW THIS IN ORDER TO CALL ATC DIRECTLY FOR FASTER RESPONSE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE ACFT'S AFT 'CHILLER'; A COOLING CABINET CHILLED BY DRY ICE IN LAYERED TRAYS; OVERHEATED AND EVAPORATED THE DRY ICE. DURING THIS OVERHEAT SIT; THERE WERE 2 BAGS OF PRETZELS IN A PARALLEL CABINET THAT EXPLODED. THIS EXPLAINS THE 'EXPLOSIVE SOUND' HEARD BY THE CABIN ATTENDANTS. IT DOES NOT EXPLAIN THE OVERHEAT OR FIRE WARNING ENSUING FROM THE REAR LAV ON THIS A319 ACFT. THE FA'S SAID THAT THERE WAS RISING SMOKE. THE VAPOR FROM THE DRY ICE SHOULD HAVE DSNDED. THE MAINT AND CFR PERSONNEL HAD REASONED THE ABOVE EVENTS AS THE PROBABLE SET OF CIRCUMSTANCES. THE CAPT; BEING UNABLE TO LOGICALLY EXPLAIN THIS WEIRD SEQUENCE OF EVENTS TO THE PAX THAT HAD TURNED INTO A MASS EXODUS OF THE ACFT; CALLED OFF DUTY FROM THE FLT AS DID THE FO. THE COMPANY TOOK THE ACFT TO THE HANGAR. THE FLT DEPARTED LATER WITH A NEW ACFT AND FLC. THE CREW WAS LATER ADVISED BY THEIR CHIEF PLT THAT THE RAMP DOES NOT HAVE THE ABILITY TO CALL THE CFR EQUIP BY RADIO. RPTR HAD HER VHF #1 SELECTOR #2 SET ON GND CTL; BUT NEVER THOUGHT TO SWITCH OVER TO IT FOR THE EMER CALL.