  Extremely bright coherent radio bursts with millisecond duration, reminiscent
of cosmological fast radio bursts (FRBs), were co-detected with
anomalously-hard X-ray bursts from a Galactic magnetar SGR 1935$+$2154. We
investigate the possibility that the event was triggered by the magnetic energy
injection inside the magnetosphere, thereby producing magnetically-trapped
fireball (FB) and relativistic outflows simultaneously. The thermal component
of the X-ray burst is consistent with a trapped FB with an average temperature
of $\sim200$-$300$ keV and size of $\sim10^5$ cm. Meanwhile, the non-thermal
component of the X-ray burst and the coherent radio burst may arise from
relativistic outflows. We calculate the dynamical evolution of the outflow,
launched with an energy budget of $10^{39}$-$10^{40}$ erg comparable to that
for the trapped FB, for different initial baryon load $\eta$ and magnetization
$\sigma_0$. If hard X-ray and radio bursts are both produced by the energy
dissipation of the outflow, the outflow properties are constrained by combining
the conditions for photon escape and the intrinsic timing offset $\lesssim10$
ms among radio and X-ray burst spikes. We show that the hard X-ray burst must
be generated at $r_{\rm X}\gtrsim10^{8}$ cm from the magnetar, irrespective of
the emission mechanism. Moreover, we find that the outflow quickly accelerates
up to a Lorentz factor of $10^2\lesssim\Gamma\lesssim10^3$ by the time it
reaches the edge of the magnetosphere and the dissipation occurs at $10^{12}$
cm $\lesssim r_{\rm radio,X}\lesssim10^{14}$ cm. Our results imply either
extremely-clean ($\eta\gtrsim10^4$) or highly-magnetized
($\sigma_0\gtrsim10^3$) outflows, which might be consistent with the rarity of
the phenomenon.
