The formation of magnetic islands with mode numbers (m>1,n=1) apparently triggered by sawtooth crashes has for a long time been a concern in JET discharges with auxiliary heating as it affects the peak performance, in terms of stored energy or of neutron yield (in DT experiments). The phenomenological association of the sawtooth crash with the onset of resistive modes is becoming particularly relevant in the context of Neoclassical Tearing Modes (NTM) theory that requires the formation of a "seed" island above a critical value. Relying on observations and on a critique of the assumed role of the collapse phase of the (1,1) instability we propose as physical mechanism of destabilisation of the metastable NTM a forced reconnection process due to electro-dynamic mode-mode coupling. For specific frequency matching conditions a bifurcation pattern appears in the Δ(W) plot. Modeling of a JET discharge shows that for perfectly frequency matching conditions the triggering of the (m=3, n=2) NTM could be due to either two-mode toroidal coupling or three-mode non-linear coupling. The observed mode frequencies are consistent with the non-linear coupling.