  We argue that giant flares in SGRs can be associated to the core conversion
of an isolated neutron star having a subcritical magnetic field $\sim 10^{12}$
G and a fallback disk around it. We show that, in a timescale of $\lesssim
10^5$ yrs, accretion from the fallback disk can increase the mass of the
central object up to the critical mass for the conversion of the core of the
star into quark matter. A small fraction of the neutrino-antineutrino emission
from the just-converted quark-matter hot core annihilates into $e^+e^-$ pairs
above the neutron star surface originating the gamma emission of the spike
while the further cooling of the heated neutron star envelope originates the
tail of the burst. We show that several characteristics of the giant flare of
the SGR 1806-20 of 27 December 2004 (spike and tail energies, timescales, and
spectra) can be explained by this mechanism.
