Sudden altitude drop in guided mode


I’ve got an F450 based drone with a Pixhawk 1, ArduCopter 4.1.5, and an attached Raspberry Pi that can execute autonomous missions in guided mode.
I was doing a scripted flight yesterday using python and the DroneKit library. The script issued a set_position_target_global_int_encode message to move towards a waypoint when suddenly the drone started descending until it landed. However, the motors kept running, not enough to lift off again but enough to make it wobble around in the grass until it flipped over and stopped.
This scripted flight has been executed several times without trouble.
One idea i have would be critically incorrect barometrical values. In fact, it was a sunny day and just before the flight the weather got cloudy, a little windy and colder. Can this have such a large impact on the measurements?
Another possibility: I leashed the drone on a long and sturdy leash. Could it be that the drone wanted to rise but was held back by the leash and interpreted this as a thrust loss (In fact, the debug message Potential Thrust Loss (4) showed up in my DroneKit log when the drone went down)?
I’m quite new to drones and the flight logs are big and scary. Maybe somebody knows where the relevant information to that problem is hidden.

Thanks in advance