  (Abridged) We propose that giant flares on Soft Gamma-Ray Repeaters produce
relativistic, strongly magnetized, weakly baryon loaded magnetic clouds,
somewhat analogous to solar coronal mass ejection (CME) events. Flares are
driven by unwinding of internal non-potential magnetic fields which leads to
slow build-up of magnetic energy outside of the neutron star. For large
magnetospheric currents, corresponding to a large twist of external magnetic
field, magnetosphere becomes dynamically unstable on \Alfven crossing times
scale of inner magnetosphere, $t_A \sim R_{NS}/c \sim 30 \mu$sec. Released
magnetic energy results in formation of a strongly magnetized, pair-loaded,
quasi-spherically expanding flux rope, topologically connected by magnetic
field to the neutron star during the prompt flare emission. Magnetic stresses
of the tied flux rope lead to late collimation of the expansion, on time scales
longer than giant flare duration. Relativistic bulk motion of the expanding
magnetic cloud, directed at an angle $\theta \sim 135^\circ$ to the line of
sight (away from the observer), results in a strongly non-spherical forward
shock with observed non-relativistic apparent expansion and bulk motion
velocities $\beta_{app} \sim \cot \theta/2 \sim 0.4 $ at times of first radio
observations approximately one week after the burst. Interaction with a shell
of wind-shocked ISM and then with the unshocked ISM leads to deceleration to
non-relativistic velocities approximately one month after the flare.
