  The pinning and collective unpinning of superfluid vortices in a decelerating
container is a key element of the canonical model of neutron star glitches and
laboratory spin-down experiments with helium II. Here the dynamics of vortex
(un)pinning is explored using numerical Gross-Pitaevskii calculations, with a
view to understanding the triggers for catastrophic unpinning events (vortex
avalanches) that lead to rotational glitches. We explicitly identify three
triggers: rotational shear between the bulk condensate and the pinned vortices,
a vortex proximity effect driven by the repulsive vortex-vortex interaction,
and sound waves emitted by moving and repinning vortices. So long as
dissipation is low, sound waves emitted by a repinning vortex are found to be
sufficiently strong to unpin a nearby vortex. For both ballistic and forced
vortex motion, the maximum inter-vortex separation required to unpin scales
inversely with pinning strength.
