Copter did TAKEOFF in stabilize mode through a mission command

I have a companion computer connected to my copter which does upload and start the mission (the mission icludes a TAKEOFF event and at the end a LAND command).
The upload to the copter did went well.
Then the companion computer does arm the the copter and once armed sends the MAV_CMD_MISSION_START command over mavlink to the pixhawk.
However somehow the flight mode change to AUTO was denied by the copter but still the copter did takeoff in stabilize mode! luckily I regained control via the transmitter and was able to safely land the copter.
Can someone tell me what exactly happend why was flight mode 3 (AUTO) rejected? And why did the copter still took off???

PS: The same way I had flown mission over 50times already this was the first time it went wrong…

2017-08-25 08-33-29.zip (553.6 KB)

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