"Disarming motors" when in mission auto mode

Hello,

I am now developing based on Copter-3.5 and testing the auto mission function with APM Planner v2.0. In SITL mode, I can command take-off and fly to a GPS waypoint in guided mode in APM.
However, I tried to add a few more waypoints and loaded in the MAVProxy with the following commands:
wp load …/Tools/autotest/ArduPlane-Missions/mission.txt
arm throttle
mode AUTO

The mission text file is as below:
QGC WPL 110
0 1 0 16 0 0 0 0 -35.3632621765136719 149.165237426757812 584.09002685546875 1
1 0 3 16 0 0 0 0 -35.3636940785166303 149.164912104606628 20 1
2 0 3 16 0 0 0 0 -35.3636328333309535 149.165684580802917 20 1
3 0 3 20 0 0 0 0 -35.3636328333309535 149.165684580802917 20 1

Then the AMP came out the “Disarming motors” message. May I know what is the reason here? Thanks.

Regards,
Eric