Crash landing with PX4 v1/11/3

This flight is for a Tarot X6 using mRo Pixhawk 2.1 that was previously flown successfully many times with ArduPilot. I converted to PX4, did all the requisite parameter settings, calibrated all sensors, did a RC range check, and did a complete inspection of the frame. Everything looked good so I put it in POSITION mode and took off. At about 15 feet it started to become unstable and was clearly not holding a GPS position. Things happened very fast after that, and I worked hard to guide it back to the ground, but just as it touched, it flipped over on its back. I tried to disarm it with the rudder stick all the way left, but the motors would not stop. Smoke started to come out of at least one motor, and props kept flailing around in the grass. The only way I could disarm was to pull the battery plug. Not a good scenario.

Looking at the logs I can see that it lost position control almost immediately. Not sure why. And I am very curious why I could not disarm it from RC transmitter.

The log file is attached. Comments please.