Helicopters have undertaken flight on their own volition all down the ages and it is not just the 350 series that has done so. Not so long ago it was an Osprey that did that as I recall.
I do admire the clairvoyance of some of you that post here.
Where did the NTSB say the Pilot was watering the tail boom?
Witnesses reported that the pilot landed and was planning on exiting the helicopter to perform a "fluid level check." After landing, the pilot exited the running helicopter; shortly thereafter the helicopter became airborne without the pilot at the flight controls. The helicopter subsequently impacted the ground and rolled over. The pilot was struck by one or more of the main rotor blades and was fatally injured.