The student pilot executed a normal landing to runway 36 in the tailwheel airplane, and the flight instructor guarded the controls. The flight instructor stated that the landing was excellent and on centerline of the runway. During rollout, a level, tail-up attitude was maintained as the airplane slowed. As soon as the tailwheel contacted the runway, the airplane began an unexpected swerve to the right. The flight instructor stated that no input from him or his student caused the swerve. Both the student pilot and the flight instructor tried to correct the swerve; however, the airplane did not respond to inputs of full left rudder and full left brake. The airplane ground looped, hit a ditch, and sustained substantial damage to the left side of the fuselage adjacent to the left wing attachment point. Examination of the airplane did not reveal any preimpact anomalies with the tailwheel assembly, main wheel brakes, or the flight controls that would have precluded normal operation. The flight instructor reported that there were no mechanical malfunctions or failures with the airplane that would have precluded normal operations. The reported wind about the time of the accident was 280° at 13 knots with gusts to 16 knots. The flight instructor estimated the wind at the time of the accident was 330° at 5 knots.