Instability of large earth structures is always a major concern in geotechnical engineering because of the potential catastrophic consequences. One of typical instability phenomena is the slope failure of flow type, characterised by a sudden loss of soil strength and a rapid movement of soil mass. Such failures have been extensively studied as static liquefaction of loose sandy soils under the undrained triaxial loading condition. However, this loading condition does not properly reproduce the field stress paths associated with rainfall in which infiltration and near-surface water flow may generate an increase in pore water pressure but almost no change in shear stress. How granular soils behave under such constant shear drained stress path and whether they exhibit association with shear behaviors under the conventional undrained and drained triaxial stress paths remain open issues. Here we present a robust grain-scale model to explore various behaviors of granular packings along different stress paths and the associated micromechanical mechanisms. Our results address from the microscopic perspectives two fundamental questions: (1) whether the behavior under the constant shear drained stress path conforms to the well-established critical state theory in soil mechanics, and (2) whether the classical Hill’s criterion works for the instability behavior under the constant shear drained stress path.