In this work we investigate the magnetic field dependence of A(g) octahedra rotation (tilt) and B-2g symmetric stretching modes frequencies at different temperatures. Our field-dependent Raman investigation at 10 K is interpreted by an ionic displacive nature of the magnetically-induced ferroelectric phase transition. The increasing frequency of the A(g) tilt mode with magnetic field gives evidence for the increase of the MnO6 tilt angle, which in turn leads to a weakening of the ferromagnetic exchange interaction, yielding the necessary conditions for the onset of ferroelectricity on the basis of the inverse Dzyaloshinskii-Moriya interaction. The frequency change of the B-2g symmetric stretching mode at the ferroelectric phase transition enables one to estimate the shift of oxygen ion positions and the corresponding spontaneous polarization of 480 mu C m(-2) in magnitude, which is of the same order as earlier reported values in single crystals.