The pilot had landed the tailwheel-equipped airplane up-slope, on a vegetation covered unimproved runway. He reported that he had landed at the same location four days prior without incident. While back taxiing on the downhill portion of the runway the airplane began to accelerate. The pilot applied the main wheel brakes in an effort to slow the airplane, but it continued into a soft muddy section, and it subsequently nosed over. The airplane sustained substantial damage to the vertical stabilizer, rudder, and left wing lift strut. The pilot stated that there were no preaccident mechanical anomalies with the airplane that would have precluded normal operation. In the recommendation section of the NTSB Accident/Incident Reporting Form 6120.1, the pilot stated that the accident may have been avoided if he had taxied at a slower speed, or if he had been more cautious of the changing runway conditions.