The mechanism of instability of flux-antiflux front in one dimensional channels in superconductors is presented. For a superconductor consists of the grains the grain boundaries form natural channels for the vortices. We study the thermal-magnetic instability in these channels. The heat, released by the vortex-antivortex annihilation is confined inside the channel due to small thermal conductivity of superconducting grains. The flux-antiflux point-like front undergoing heating creates a normal spot inside the channel which can dramatically change the voltage current characteristic transforming it to a non Bardeen-Stephen shape. We present the conditions of  the front stability for various temperatures and heating parameters. We predict that heat released by the vortex-antivortex annihilations enhances the instability in superconductors with grain boundaries causing a jump in the voltage. The results are in a good agreement with recent experiments.