  The recent discovery of a Galactic fast radio burst (FRB) occurring
simultaneously with an X-ray burst (XRB) from the Galactic magnetar SGR
J1935+2154 implies that at least some FRBs arise from magnetar activities. We
propose that FRBs are triggered by crust fracturing of magnetars, with the
burst event rate depending on the magnetic field strength in the crust. Crust
fracturing produces Alfv\'en waves, forming a charge starved region in the
magnetosphere and leading to non-stationary pair plasma discharges. An FRB is
produced by coherent plasma emission due to nonuniform pair production across
magnetic field lines. Meanwhile, the FRB-associated XRB is produced by the
rapid relaxation of the external magnetic field lines. In this picture, the
sharp-peak hard X-ray component in association with FRB 200428 is from a region
between adjacent trapped fireballs, and its spectrum with a high cutoff energy
is attributed to resonant Compton scattering. The persistent X-ray emission is
from a hot spot heated by the magnetospheric activities, and its temperature
evolution is dominated by magnetar surface cooling. Within this picture,
magnetars with stronger fields tend to produce brighter and more frequent
repeated bursts.
