A mechanism for the onset of the major disruption in tokamaks is proposed which considers the effect of magnetic stochasticity on the growth of the m=2 tearing mode. 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 the rapid growth are studied. The role of the toroidal coupling to the m=1 component is also discussed