A320 FLT CREW EXPERIENCES UNCOMMANDED BRAKING AFTER A POWER SHIFT DURING TAXI.

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DURING BACK-TAXI ON RWY 16; WE EXPERIENCED AN UNCOMMANDED BRAKING INPUT DUE TO A PWR XFER OF AN APU ONLINE. THE PURSER RPTED BOTH SHE AND ANOTHER FLT ATTENDANT WERE CONCERNED THE RESULTANT MOTION MAY HAVE AGGRAVATED PREVIOUS INJURIES FROM WHICH THEY HAD RECOVERED. BOTH FLT ATTENDANTS STATED THEY WERE ABLE TO PERFORM THEIR DUTIES WITHOUT LIMITATION FOR THE RETURN FLT; THOUGH THEY WOULD EVALUATE THEIR CONDITION OVER THE NEXT FEW DAYS. BOTH STATED THEY WERE STRAPPED INTO THEIR JUMP SEATS AT THE TIME OF FEELING THE BRAKING ACTION. THE ARPT ENVIRONMENT REQUIRED BACK-TAXI AFTER LNDG DUE TO HAVING NO PARALLEL TXWYS. WE FELT PRESSURE TO EXPEDITE CLRING THE RWY AS WE WERE NOSE-ON TO 2 SETS OF LNDG LIGHTS. 1 ACFT WAS CLRED ONTO HOLD; AND ANOTHER WAS ON FINAL. I WAS FOCUSED ON CLRING THE RWY IN TIME WHILE THE FO WAS CONFIGURING FOR A SINGLE ENG TAXI IN. THE FO QUERIED ME TO CONFIRM MY NOT MAKING BRAKE OR STEERING INPUTS. AT FIRST I TOLD HIM TO STAND BY UNTIL I COULD LOWER MY SPD NEAR THE EXIT. ON SLOWING TO MAKE THE EXIT; I CONFIRMED I WAS MAKING NO INPUTS; AND THE FO DESELECTED THE #2 GENERATOR. THE APU THEN ACHIEVED OPERATING SPD; ITS GENERATOR CAME ONLINE WHILE I REAPPLIED BRAKES; CREATING THE UNCOMMANDED BRAKE INPUT. THIS OCCURRED AT 15-20 KTS; AND THE INPUT LASTED MAYBE HALF A SECOND. NEITHER OF US CONFIRMED THE APU GENERATOR WAS DESELECTED OR THE APU HAVING ACHIEVED OPERATING SPD PRIOR TO DESELECTING THE #2 GENERATOR. EITHER OF THESE CONDITIONS WOULD HAVE PREVENTED THE UNCOMMANDED BRAKE INPUT. ADDITIONAL FACTORS WERE: A REQUIREMENT TO START THE APU ASAP AFTER LNDG TO FACILITATE A BLOCK-IN WITHOUT GND PWR; THE RAMP WAS CONGESTED AND IN CLOSE PROX TO THE RWY EXIT.