The operator reported that the pilot taxied the high-wing airplane from the ramp along the parallel taxiway to the active runway (27) with a quartering tailwind. He reported that when the pilot initiated a turn to the right at the taxiway intersection, the airplane did not respond to the control inputs and continued straight ahead. The pilot applied full right brake and rudder, however; the airplane continued to track forward and exited the hard surface taxiway. The airplane continued off the taxiway and traveled an additional 80 feet through rough terrain before it came to rest in a nosed down attitude. The airplane sustained substantial damage to the right wing. Winds at the time of the accident were from 260 degrees at 27 knots, gusting to 38 knots. No pre accident mechanical malfunctions or failures were reported with the control system or brakes.