The plane comes over the trees in Autonomous mode.
It descends towards the ground.
The Sonar sees the ground, and rapidly stops the descent, the plane flys parrallel to the ground at about 1 meter.
The plane then lands.
The last bit, "the plane then lands", was not actually meant to happen and is now under technical invtestigation from the telemetry.
I have explained why the plane landed (rather than go around), in this thread on the uavdevboard-dev forum
Project: MatrixPilot / UAV Devboard.