We present full-particle simulations of 2-D magnetotail current sheet equilibria with open boundaries and zero driving. The simulations show that spontaneous formation of dipolarization fronts and subsequent formation of magnetic islands are possible in equilibria with an accumulation of magnetic flux at the tailward end of a sufficiently thin current sheet. These results confirm recent findings in the linear stability of the ion tearing mode, including the predicted dependence of the tail current sheet stability on the amount of accumulated magnetic flux expressed in terms of the specific destabilization parameter. The initial phase of reconnection onset associated with the front formation represents a process of slippage of magnetic field lines with frozen-in electrons relative to the ion plasma species. This non-MHD process characterized by different motions of ion and electron species generates a substantial charge separation electric field normal to the front.