I have taken a deeper look at the flight log and my conclusion is that the sudden increase in the speed of the craft was due to the craft switching to the ATTI mode in which the speed change is a lot more sensitive to stick input compared with that in tripod mode ( or C mode ) which was selected throughout the flight.
This chart summarizes the forward/backward ( or elevator ) control applied. Basically you flew the craft out by appling forward stick and then back by backward stick. The orientation of the craft has not changed much , ie, the nose kept facing the river.
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The next chart shows what caught you by surprise. The GPS signal was too weak to to be usable and the craft just switched between vision and ATTI mode. In vision mode, the craft braked automatically when the stick was released as indicated by the black circles.
For Mini the max speed in C mode is limited to 4 m/s which corresponds to a maximum steady tilt of about 7 degrees as shown in the earlier part of the flight. However, in ATTI mode, the tilt is much bigger so if the craft switches from C mode to ATTI mode under full elevator input, the tilt will suddenly jump and the speed abruptly increase. This has occurred twice in the later part of the flight. In the first occurrence you released the stick immediately and by coincidence the craft switched back to C mode at the same time and braked automatically. No harm done. Then after about 5 seconds later it happened again and you applied full counter elevator input but unfortunately the craft switched back to C mode and could not tilt enough to stop in time. Collision followed immediately.
View attachment 119168