Unplanned landing

We were checking our device during the last flight. At the end of it we had a strange situation. So, the flight was in automatic mode and we had a connection with “Ground Control Station”, but not with “Radio Control”. We were planning to fly about 20 kilometers, however for unknown reasons our apparatus flew just 13.5 kilometers, stopped and made a very fast landing (12 meters per second of a maximum vertical speed).
See the files attached: .log file and .param file