The commercial pilot/registered owner was standing on the ramp in his single-engine airplane with the engine running and the parking brake set. As the pilot was preparing to taxi, he dropped something on the floor. When he realized he could not reach the object, he moved his seat back so he could reach down to the floor. At that point, he felt motion and realized that he could not reach the brake pedals. Before the pilot could release his seatbelt and turn off the engine, the airplane struck two parked airplanes, resulting in minor damage to his airplane and one of the parked airplanes. However, the other parked airplane sustained substantial damage to the rudder, and horizontal stabilator.