LCL CTLR AT IAD ISSUED TAXI INSTRUCTIONS WHILE LNDG ACFT IS IN DEEP REVERSE PITCH.

Narrative

AFTER LNDG ON RWY 16 AT DULLES; WE CLRED THE RWY AT THE FIRST HIGH SPD TXWY IN ORDER TO HELP THE TWR OUT WITH ACFT SPACING BEHIND US. AS WE WERE IN DEEP REVERSE (WHICH IS QUITE LOUD) AND STILL ON THE RWY; TWR GAVE INSTRUCTIONS TO CLR THE RWY AND TO CONTACT GND. NORMALLY AT DULLES; TWR SAYS TO CONTINUE TAXIING AND TO CONTACT GND. WITH THE LOUD NOISE FROM REVERSE PITCH; WE BOTH THOUGHT THAT IS WHAT THE TWR SAID TO DO. SO I CONTINUED TO TAXI ON THE PARALLEL TXWY AND CALLED GND. GND INFORMED US THAT THE TWR'S INTENTION WAS FOR US TO HOLD SHORT OF THE PARALLEL TXWY AND THEN CALL GND FOR FURTHER CLRNC. IT WAS INCORRECT OF US TO ASSUME THAT WE WERE NOT TO HOLD SHORT OF THE PARALLEL TXWY; AND WE SHOULD HAVE ASKED FOR CLARIFICATION AFTER CLRING THE RWY. A CONTRIBUTING FACTOR IS DULLES' BAD HABIT OF ISSUING TAXI INSTRUCTIONS IMMEDIATELY AFTER TOUCHDOWN; WHICH I STILL CONSIDER TO BE A CRITICAL PHASE OF FLT. ALSO; THIS WAS THE 6TH AND LAST LEG OF A 12 HR DUTY DAY; SO FATIGUE MAY HAVE BEEN A FACTOR.