PLT OF C182 OPERATING IN DC ADIZ INADVERTENTLY SWITCHED TO 1200 SQUAWK WHEN TERMINATED FROM RADAR SVC WHILE LNDG AT JYO.

Narrative

I VIOLATED THE WASHINGTON BALTIMORE ADIZ BY RESETTING MY SQUAWK CODE FROM THE ASSIGNED CODE I RECEIVED PRIOR TO ENTERING THE ADIZ TO 1200 DURING MY APCH TO LEESBURG; VA; ARPT FOR LNDG. I RESET THE CODE AFTER RECEIVING ATC COMS TERMINATING RADAR SVC AND APPROVING FREQ CHANGE TO THE LEESBURG CTAF. UPON LNDG; I WAS INFORMED OF THE INFRACTION BY POTOMAC TRACON THERE IS NO QUESTION BUT THAT I RESET THE CODE. AT THE SAME TIME; I AM VERY AWARE OF THE ADIZ PROC TO MAINTAIN THE ASSIGNED CODE WITHIN THE ADIZ UNTIL LNDG. I HAVE REPLAYED THE EVENTS NUMEROUS TIMES AND CANNOT COME UP WITH A REASON OTHER THAN SIMPLY MAKING A HABITUAL PROC ASSOCIATED WITH THE TERMINATION OF RADAR SVC. I HAVE REQUESTED A COPY OF THE ATC RECORDINGS BTWN THE CTLR AND MYSELF TO SEE IF THERE WAS ANYTHING IN THE COMS THAT PROMPTED ME TO RESET THE XPONDER WHEN I DID. USUAL ATC COMS IN THE ADIZ INCLUDE INSTRUCTIONS TO MAINTAIN THE CODE UNTIL ON THE GND. HOWEVER; I CANNOT RECALL WHETHER THAT PHRASE WAS OR WAS NOT INCLUDED IN THIS INSTANCE.