the tail assembly was oscillating a good 30 cm in each direction
Perhaps not quite that much, albeit the old girl can put on a good shake, rattle, and roll show ?
Any structure will vibrate if you hit it with some excitation at the the right sort of frequency. Nomad is no different in that regard. One relevant difference is that the tail assembly oscillates at frequencies low enough to see easily during ground run ops.
Years ago I did a video comparison between Nomad and a number of other aircraft on high power ground ops with no undue surprises surfacing.