Going back to the 737 problems, I'd throw in with the idea that the yaw damper is causing a 'chattering' hydraulic actuator. The damage to the A-300 rudder - versus the fin - rather suggests radical flutter, which induced the failure of the vertical fin attachments. Im guessing a case of 'deuling' actuators took place.
A number of 737 hardover events have been solved by turning off the yaw damper. While that's inconclusive, I wouldn't be too quick to discount the possibility.