  The macroturbulence instability observed in fluxline systems during
remagnetization of superconductors is explained. It is shown that when a region
with flux is invaded by antiflux the interface can become unstable if there is
a relative tangential flux motion. This condition occurs at the interface when
the viscosity is anisotropic, e.g., due to flux guiding by twin boundaries in
crystals. The phenomenon is similar to the instability of the tangential
discontinuity in classical hydrodynamics. The obtained results are supported by
magneto-optical observations of flux distribution on the surface of a YBCO
single crystal with twins.
