What is the Tacoma Narrows bridge doing in this picture?

When the plate of the bridge assumes the unnatural shape showed in the following picture, what is this phenomenon called? Contortion (example: the bridge was contorting ...). I'm not so sure, as I think contortion normally refers to the (unnatural) shape of human/animal body. Is there a better word?

This bridge is thrashing or maybe even convulsing.

I'm adding to this question at this late date because I think the answers to date have been overly restricted to technical (i.e., 'scientific') answers. When someone keels over clutching their chest, one is much likelier to hear "Oh my god, she's having a heart attack!" instead of the more medical "Oh my god, she's having a myocardial infarction!"

While contortion may sometimes refer to the shape of a human/animal body, it does not necessarily.

Contortion and torsion both refer to the twisting (unnatural in the case of contortion) of an object, which certainly seems relevant for the photo you provided.

The phenomenon in question actually deals with the resonance of the bridge, and is not really a "twisting" motion as contortion implies, but is actually an oscillation due to resonant frequencies of the object. (Though it certainly appears as a twisting motion when driven to that type of extreme.)

As pointed out by others and in comments, aeroelastic flutter played the major roll and not resonance. Though it remains an example of resonance, calling it resonance oversimplifies what actually occurred and it would be more accurate to state that it was aeroelastic flutter combined with resonance.(source1)(source2)(source3)(source4)