1999 HONDA ACCORD — POWER TRAIN:AUTOMATIC TRANSMISSION:PARK/NEUTRAL START INTERLOCK SWITCH

I PULLED MY ACCORD UP TO THE FRONT DOOR OF A SUBWAY SHOP. I SET THE BRAKE, REMOVED KEYS AND PUT MY CELLULAR PHONE IN MY TRUNK. NO MOVEMENT OF CAR TOOK PLACE. I WENT IN THE SHOP TO ORDER MY SANDWICH. I SAT DOWN TO EAT---THE VEHICLE WAS STILL IN PLACE.AS I WAS DISPOSING OF TRASH. WHEN I WENT TO GET IN VEHICLE, IT WAS GONE.I THOUGHT SOMEONE GOT ANOTHER ACCORD.I LOOKED 100 FEET TO MY LEFT AND THE CAR WAS WEDGED BETWEEN TWO CARS.THE KEYS WERE IN MY POCKET.THE TWO OWNERS OF THE DAMAGED VEHICLES COULD NOT BELIEVE WHAT HAPPENED.ONE OWNER, AN INSURANCE BROKER, SAID THIS WAS A FIRST FOR HIM. MY QUESTION : WHY WOULD A VEHICLE ROLL WHEN THE IGNITION WAS LOCKED? *AK