Multiplicity of phase states within frequency locked bands in periodically forced oscillatory systems may give rise to front structures separating states with different phases. A new front instability is found within bands where ω_forcing/ω_system=2n (n>1). Stationary fronts shifting the oscillation phase by π lose stability below a critical forcing strength and decompose into n traveling fronts each shifting the phase by π/n. The instability designates a transition from stationary two-phase patterns to traveling n-phase patterns.