  Within linear continuum theory, no magnetic texture can propagate faster than
the maximum group velocity of its spin waves. Here we report a transient regime
due to the appearance of additional antiferromagnetic textures that breaks the
Lorentz translational invariance of the magnetic system by atomistic spin
dynamics simulations. This dynamical regime is akin to domain wall
Walker-breakdown in ferromagnets and involves the nucleation of an
antiferromagnetic domain wall pair. Subsequently, one of the nucleated
180$^{\circ}$ domain wall creates with the original domain wall a 360$^{\circ}$
spin-rotation which remains static even under the action of the spin-orbit
field. The other 180$^{\circ}$ domain wall becomes accelerated to
super-magnonic speeds. Under large spin-orbit fields, multiple domain wall
generation and recombination is obtained which may explain the recently
experimentally observed current pulse induce shattering of large domain
structures into small fragmented domains and the subsequent slow recreation of
large-scale domain formation prior current pulse.
