The pilot of the tailwheel-equipped airplane had spent the morning practicing hilltop landings with a second airplane, as a flight of two. The hilltop where the accident occurred was the third landing location. The landing was uphill and uneventful, and after coming to a stop he parked the airplane, exited, and went for a hike. After returning, the first airplane departed downhill successfully, with the accident airplane in trail. The pilot reported that once airborne, the airplane drifted left, and did not respond fully to his control inputs. He further stated that a slight upslope air movement was present, and that the slipstream from the propeller of the leading airplane may have contributed to the loss of control. The left wingtip struck a tree and the airplane tumbled. The pilot reported no mechanical malfunctions or failures with the airplane that would have precluded normal operation. The airplane sustained substantial damage to both wings and the forward cabin during the accident sequence.