This paper investigates the mechanical conditions governing the activation of shallow landslides in unsaturated deposits. Particular attention is given to the effect of saturation processes applied under a constant total state of stress. For the sake of generality, multiple failure mechanisms are considered and are interpreted in the light of a unified theoretical framework. The analyses rely on the scheme of infinite slope and on the use of a coupled hydro-mechanical constitutive model. Simple shear test simulations are used to investigate the role of stress anisotropy, initial suction and soil properties. The simulations show that the predicted mechanism of activation depends on the material constants that reproduce the coupling between retention properties and stress-strain behavior. These constants affect the magnitude of the triggering perturbations and can play a relevant role in collapsible/liquefiable deposits. In particular, it is shown that the range of slope inclinations prone to originate a runaway instability can be a function of such hydro-mechanical coupling terms, thus depending on material constants that are not directly associated with the shearing resistance.