  Bright outbursts from Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars
(AXPs) are believed to be caused by instabilities in ultramagnetized neutron
stars, powered by a decaying magnetic field. It was originally thought that
these outbursts were due to reconnection instabilities in the magnetosphere,
reached via slow evolution of magnetic footpoints anchored in the crust. Later
models considered sudden shifts in the crust's structure. Recent observations
of magnetars give evidence that at least some outburst episodes involve
rearrangements and/or energy releases within the star. We suggest that bursting
episodes in magnetars are episodes of rapid plastic yielding in the crust,
which trigger "swarms" of reconnection instabilities in the magnetosphere.
Magnetic energy always dominates; elastic energy released within the crust does
not generate strong enough Alfven waves to power outbursts. We discuss the
physics of SGR giant flares, and describe recent observations which give useful
constraints and clues.
