Magnetic triggering of edge localized modes (ELMs) in Ohmic H-mode plasmas was first reported in the TCV tokamak (Degeling et al 2003 Plasma Phys. Control. Fusion45 1637). This method, showing reliable locking of the ELM frequency to an imposed axisymmetric vertical plasma oscillation, was also demonstrated in the ITER-relevant type-I ELM regime in ASDEX Upgrade (Lang et al 2004 Plasma Phys. Control. Fusion46 L31) and JET (de la Luna et al 2015 Nucl. Fusion56 026001). However, the mechanisms of the ELM triggering due to a vertical motion has not been studied extensively. The non-linear reduced MHD code JOREK-STARWALL has been extended for 3D free-boundary computations (Hölzl et al 2012 J. Phys.: Conf. Ser. 401 012010), which has allowed us to simulate for the first time realistic vertical oscillations together with ELM simulations in a single consistent scheme. Our simulations demonstrate that stable plasmas can be destabilized by the application of a vertical oscillation for ITER. During the vertical motion, a toroidal current is induced in the pedestal. The origin of this current is analysed in detail with the use of simulations and a simple analytical model, revealing that it arises from the compression of the plasma cross section due to its motion through an inhomogeneous magnetic field. Lower pedestal currents between ELMs require bigger vertical displacements to destabilize ELMs, which directly points towards the increased edge current as the ELM driving mechanism. Finally the ELM triggering shows a very weak dependence on the plasma velocity for ITER in agreement with experiments.