From a mechanical engineer's perspective, it is correct to say that the bridge is contorting.
I would comment on some of the previous posts, but I am new to this forum.
Although not specifically asked for, many others are identifying the cause of this torsion as resonance; however, this is not technically correct. As Shyam points out, the bridge failed due to a phenomenon called aeroelastic flutter.
Resonance is a technical term which implies that a periodic force was applied at the same frequency of the structure's natural frequency. The wind blowing on the Tacoma Narrows Bridge was not periodic in nature: it was a constant breeze. Thus, resonance is not technically correct here.