KD is on the right track.
There is a Bulletin in effect to alleviate shaft bowing; after the cool down cycle (approx 3 mins) the APU switch is turned to the ON position (not START) to open the APU inlet door. After 40 mins the switch should be turned to OFF to close the door. If takeoff occurs during the 40 min wait, the APU switch can be turned off at a convenient time after takeoff.