Why does gravitational radiation produce vorticity?

We calculate the vorticity of world–lines of observers at rest in a Bondi–Sachs frame, produced by gravitational radiation, in a general Sachs metric. We claim that such an effect is related to the super–Poynting vector, in a similar way as the existence of the electromagnetic Poynting vector is related to the vorticity in stationary electrovacum spacetimes.