  We examine two trigger mechanisms, one internal and the other external to the
neutron star, that give rise to the intense soft gamma-ray repeater (SGR) giant
flares. So far, three giant flares have been observed from the three out of the
seven confirmed SGRs on March 5, 1979, August 27, 1998, and December 27, 2004.
The last two events were found to be much more powerful than the first, and
both showcased the existence of a precursor, that we show to have had initiated
the main flare. In the internal mechanism, we propose that the strongly wound
up poloidal magnetic field develops tangential discontinuities and dissipates
its torsional energy in heating the crust. The timescale for the instability to
develop coincides with the duration of the quiescent state that followed the
precursor. Alternatively, we develop a reconnection model based on the
hypothesis that shearing motion of the footpoints causes the materialization of
a Sweet-Parker current layer in the magnetosphere. The thinning of this
macroscopic layer due to the development of an embedded super-hot turbulent
current layer switches on the impulsive Hall reconnection, which powers the
giant flare. Again, we show that the thinning time is on the order of the
preflare quiescent time. This model naturally explains the origin of the
observed nonthermal radiation during the flares, as well as the post flare
radio afterglows.
