  A luminous radio burst was recently detected in temporal coincidence with a
hard X-ray flare from the Galactic magnetar SGR 1935+2154 with a time and
frequency structure consistent with cosmological fast radio bursts (FRB) and a
fluence within a factor of $\lesssim 10$ of the least energetic extragalactic
FRB previously detected. Although active magnetars are commonly invoked FRB
sources, several distinct mechanisms have been proposed for generating the
radio emission which make different predictions for the accompanying higher
frequency radiation. We show that the properties of the coincident radio and
X-ray flares from SGR 1935+2154, including their approximate simultaneity and
relative fluence $E_{\rm radio}/E_{\rm X} \sim 10^{-5}$, as well as the
duration and spectrum of the X-ray emission, are consistent with extant
predictions for the synchrotron maser shock model. Rather than arising from the
inner magnetosphere, the X-rays are generated by (incoherent) synchrotron
radiation from thermal electrons heated at the same shocks which produce the
coherent maser emission. Although the rate of SGR 1935+2154-like bursts in the
local universe is not sufficient to contribute appreciably to the extragalactic
FRB rate, the inclusion of an additional population of more active magnetars
with stronger magnetic fields than the Galactic population can explain both the
FRB rate as well as the repeating fraction, however only if the population of
active magnetars are born at a rate that is at least two-orders of magnitude
lower than that of SGR 1935+2154-like magnetars. This may imply that the more
active magnetar sources are not younger magnetars formed in a similar way to
the Milky Way population (e.g. via ordinary supernovae), but instead through
more exotic channels such as superluminous supernovae, accretion-induced
collapse or neutron star mergers.
