  Solar tornados are dynamical, conspicuously helical magnetic structures
mainly observed as a prominence activity. We investigate and propose a
triggering mechanism for the solar tornado observed in a prominence cavity by
SDO/AIA on September 25, 2011. High-cadence EUV images from the SDO/AIA and the
Ahead spacecraft of STEREO/EUVI are used to correlate three flares in the
neighbouring active-region (NOAA 11303), and their EUV waves, with the
dynamical developments of the tornado. The timings of the flares and EUV waves
observed on-disk in 195\AA\ are analyzed in relation to the tornado activities
observed at the limb in 171\AA. Each of the three flares and its related EUV
wave occurred within 10 hours of the onset of the tornado. They have an
observed causal relationship with the commencement of activity in the
prominence where the tornado develops. Tornado-like rotations along the side of
the prominence start after the second flare. The prominence cavity expands with
acceleration of tornado motion after the third flare. Flares in the
neighbouring active region may have affected the cavity prominence system and
triggered the solar tornado. A plausible mechanism is that the active-region
coronal field contracted by the `Hudson effect' due to the loss of magnetic
energy as flares. Subsequently the cavity expanded by its magnetic pressure to
fill the surrounding low corona. We suggest that the tornado is the dynamical
response of the helical prominence field to the cavity expansion.
