I am concerned that several pilots seem genuinely baffled that rudder use could have the fin off a fully serviceable aircraft that was properly designed and certificated. Unhappily it can, as I will try to explain in a moment.
Before I go on please may I emphasise I am not suggesting this caused the accident this thread is about, but offer these comments to the general debate about rudder use that is going on.
The mechanism I refer to is the same one employed by adults when helping youngsters to enjoy a swing. The adult applies a very small force at just the right moment in the cycle and by so doing builds up the oscillation until in the end the displacement of the swing can become very large indeed (with much yelping from the occupant)
A similar relatively small side force generated by rudder deflection, repeatedly applied, can cause a yaw oscillation to build rather than damp. There are two possible end points in this case, either the fin will stall due to the size of its AoA (in which case the aircraft will depart from controlled flight) or the fin will break before it stalls due to aerodynamic overload.
We all enjoy the powerful damping effects of yaw autostabilisers when they produce a very small force and apply it at the right moment to reduce the yaw oscillations. Now imagine a yaw autostabiliser that is working in reverse. Or a pilot that is out of phase with his feet. Or a pilot that is pushing the rudder pedals correctly but control actuation lags result in those correct inputs being delayed to an incorrect time.