Recently, a bright coherent radio burst with millisecond duration, reminiscent of cosmological fast radio bursts (FRBs), was co-detected with an anomalously-hard X-ray burst from a Galactic magnetar SGR 1935$+$2154. We investigate the possibility that the event was triggered by a deposition of a magnetic energy in a localized region of the magnetosphere, thereby producing a so-called trapped fireball (FB) and simultaneously launching relativistic outflows. We show that the thermal component of the X-ray burst spectrum is consistent with a trapped FB with an average temperature of a few hundred keV and a 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 $sim10^{39}mbox{-}10^{40}$ erg comparable to that of the trapped FB, for a variety of baryon load $eta$ and initial magnetization $sigma_0$ parameters. If both the hard X-ray and radio bursts are produced by the energy dissipation of the outflow, the properties can be constrained by the conditions for photon escape and the intrinsic timing offset of $lesssim 10$ ms among the radio and X-ray burst spikes. We show that the hard X-ray bursts need to be generated at $r_{rm X}gtrsim10^{8}$ cm from the stellar surface, irrespective of the emission mechanism. Particularly in the case of shock dissipation, the outflow should accelerate up to a Lorentz factor of $Gamma gtrsim 10^3$ by the time it reaches the outer edge of the magnetosphere and the shock dissipation should take place at $10^{12},mathrm{cm} lesssim r_{rm radio, X} lesssim 10^{14},mathrm{cm}$. In this case, extremely clean ($etagtrsim10^4$) and/or highly magnetized ($sigma_0gtrsim10^3$) outflows are implied, which may be consistent with the rarity of this phenomenon.
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