Heavy rainfall can lead to shallow slips in slopes that are initially in a state of partial water saturation. Multiphysics numerical modelling approaches taking into account the involved physical key processes in variably saturated soils during rainfall events could help in understanding the main slip mechanisms. The concerned processes are related to water flow through the solid matrix, soil water retention behaviour and the effects of matric suction on the mechanical behaviour. In this paper, the elasto-plastic constitutive model ACMEG-s that captures some key features of the behaviour of variably saturated soils is used in a fully coupled hydromechanical finite element analysis for the assessment of destabilizing, transient processes in a steep slope during rain infiltration. It is shown that at the onset of failure, wetting pore collapse and plastic shear strains occur in the lower part of the slope and develop upwards towards the slope surface to delimit a probable failure mechanism.