A mechanism for the onset of the major disruption is proposed which considers the effect of magnetic stochasticity on the growth of the m=2 tearing mode. The toroidicity can cause stochasticity near the separatrix of the m=2 magnetic island which enhances the current viscosity, resulting in explosive growth. The threshold condition and the time scale of rapid growth are studied. The role of the toroidal coupling to the m=1 component is also discussed. (author).