PAX DOOR OPENED DURING TKOF CAUSING THE FLC TO RETURN AND LAND.

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INST STUDENT AND INSTRUCTOR RECEIVED CLRNC AND TAXI INSTRUCTIONS FOR RWY 22R. TAXI AND RUNUP WAS UNEVENTFUL AND PERFORMED WHEN REACHING RWY HOLD SHORT AREA; AS #2 OR #3 TO GO. FINAL CHK 'ON AND HOLD' WITHOUT PROB. WE WERE ALERT TO POSSIBLE WINDSHEAR FROM DEPARTING ACFT; CLRED TO GO. WE ACCELERATED AND ROTATED. AT APPROX 50 FT; INSTRUCTOR NOTICED SUDDEN CABIN CHANGE IN NOISE AND TEMP; AS MAIN DOOR HAD FOR UNKNOWN REASON BECOME UNSECURED BY TOP LATCH. WE CTLED ACFT; AND PRIOR TO SWITCH TO DEP REQUESTED A NON PRIORITY RETURN DUE TO OPEN DOOR. REQUEST WAS GRANTED. CIRCLED AND RETURNED UNEVENTFULLY. RETURNED TO FBO TO EXAMINE CAUSE. NOTE: ACFT HAD ALREADY FLOWN 4.5 HRS WITH 3 STOPS; WITH NO IDENTICAL PROBS. AFTER REATTEMPTING 3 CLOSURES SUCCESSFULLY BACK-TO-BACK; NO CAUSE DETERMINED. RETURNED FOR TKOF AND DEPARTED WITHOUT INCIDENT. ONLY POSSIBLE IDEA: FIGHTING STRONG R XWINDS AT ROTATION; FORCING INSTRUCTOR TO LEAN SIGNIFICANTLY TOWARDS R. PERHAPS THIS PRESSURE ON INSIDE OF DOOR WAS TOO MUCH FOR UPPER LATCH.