After a normal touchdown a swing to the right developed. The swing was controlled initially with rudder, but when the brakes were applied the pilot was unable to maintain directional control. After the aircraft had swung through about 120 degrees the left wheel was dislodged. The left main axle assembly had separated from the leg because a bolt had failed and a section of the broken bolt had jammed between the brake unit and the brake disc, locking the left wheel. The locked wheel caused the remaining bolts to fail. The bolt that failed was found to have been worn by fretting. The aircraft had only operated 20 hours since servicing was carried out to the gear legs. The failed bolt had not been changed at that service.