A new mechanism is proposed that is able to explain the sudden disruption of the magnetotail energy storage and the explosive onset of isolated magnetospheric substorms. During the growth phase the magnetotail accumulates additional magnetic flux and evolves in such a way that the special parameter &kgr;e=(Bn/Bo)(L/&rgr;eo)1/2 describing the ratio of characteristic frequencies of the electron motion along and across the field reversal region quickly decreases. Here Bn and Bo are the components of magnetic field across and along the central plasma sheet, L is the thickness of the field reversal region, and &rgr;eo is the Larmor radius of thermal electrons in the field Bo. When &kgr;e reaches a critical value &kgr;e‖crit≂1.6, a nonlinear interaction between the degrees of freedom of electron motion becomes strong enough to cause a chaotization of the electron orbits. This effect results in a fast growth of a collisionless tearing mode instability even within the framework of the WKB approximation, i.e., in a wide interval of possible wavelengths 1>kL>Bn/Bo. The proposed theory gives the mechanism of the formation of new reconnection region in the near-Earth magnetotail, based exclusively on intrinsic properties of the magnetotail evolution. This reveals the natural relation between the onsets of isolated substorms and an internal large-scale magnetotail instability of the tearing type considered in this paper. ¿ American Geophysical Union 1987