This work discusses a possible mechanism of spontaneous, triggerless onset of NTMs, observed sometimes on JET, associated with changes in the plasma rotation. The NTM onset threshold is associated with the ion polarisation current: this current can be either stabilizing or destabilizing depending on island rotating in the ion or electron diamagnetic drift direction, and is dependent on to the viscous regimes. Since in the nonlinear stage the magnetic islands necessarily break the ideal tokamak axisymmetry, a Neoclassical Toroidal Viscosity (NTV) appears in the toroidal direction, eventually determining self-consistently the magnetic island propagation speed.