If you are logging and it happens with logging enabled, there will be some things in it that can be looked at.
RC inputs will show bad values, the flight mode will change, etc.
You can experiment with this by switching off the transmitter.
Look in the System -> DataObjects, in ManualControlCommand (Channel and FlightModeSwitchPosition). You may want to increase the logging rate for these things, but careful to not over do it or it will swamp the telemetry channel.
For a test you might reconfigure the FC failsafe to be Attitude mode with throttle a little below hover power (on a fully charged battery). Careful. If you set this too high and loose RC signal it will fly away. Also, the motors will continue to spin after landing and that means props chewing things on the way down, props tangled in things and burned motors and ESCs and starting a fire. If it is an RC link issue and that causing an FC failsafe, this will change it from "drop" to "come down slowly" and you will at least know that is your problem.