I step into this discussion with much trepidation <img src="confused.gif" border="0"> so be gentle on me if I'm wrong, but...
I can remember getting a phenomina called "Fin-Stall" when I've mis-handled the rudder during aero's or, I think, E/O exercises in a twin turbo-prop. With me so far?
At the speed these guys were flying SURELY the fin would have stalled out before the side-loading exceeded the permissable or sensible structural limit and prevented the seperation.
At higher speeds Boeings (some) have rudder limiters to prevent excessive loading on the fin from over-exuberant application of rudder.
Any comments?
If Deadflyer's report about the oscillations is correct then look for component failure, 5 full cycles in less than 5 seconds is mishandling even I'm incapable off!!