This model explains why breakup has been observed as low as L = 5-6 and predicts that a higher concentration of O+ in the plasma sheet will permit breakup at such low L values. It is argued that the establishment of a coupled current structure within a single arc leads to a quasi-stable system i.e. the pre-breakup regime. Perturbation of the pre-breakup structure leads to an instability criterion. Certain quiescent configurations are inherently unstable. Thus transitions from a stable to an unstable situation can trigger a rapid poleward expansion of the quiescent arc or substorm breakup. In our model the kinetic energy of injected plasma from the magnetotail supplies the energy for breakup.