A JOREK 3D non-linear MHD simulation of a disruption triggered by an argon massive gas injection in JET, which quantitatively reproduces the plasma current () spike (Nardon et al 2021 Plasma Phys. Control. Fusion63 115006), is analyzed in order to investigate the origin of the spike and its relation with magnetic stochasticity. The spike is associated to a current density (jφ) profile relaxation which appears to result from Shear Alfvén Wave (SAW) propagation along stochastic field lines, as proposed by Boozer (2019 Plasma Phys. Control. Fusion61 024002; 2020 Phys. Plasmas27 102305), possibly complemented by a macroscopic E×B flow structure. Using axisymmetric JOREK simulations involving a mean field Ohm's law, we verify that the level of hyper-resistivity associated to SAWs is consistent with the prediction made in (Boozer 2019 Plasma Phys. Control. Fusion61 024002; Boozer 2020 Phys. Plasmas27 102305), which connects the spike with the level of stochasticity. The relaxation comprises two main phases, the first one corresponding to a fast (0.1 ms) and almost complete jφ flattening in the q < 2 region, while the second one is longer (0.5 ms) and corresponds to a more gradual, global and incomplete jφ flattening. During the first phase, strong E×B flows develop that play a key role in mixing impurities into the core.