We propose a mechanism previously developed as a hypothetical cause of the initiation of subduction in the Earth's mantle, to describe a situation where such subduction may occur transiently, at irregular intervals of time. It has been suggested that tectonics on Venus may be described by such a scenario. In our model, a subduction event is followed by resumption of high Rayleigh number mantle convection below a stagnant lithosphere which thickens due to conductive cooling. As it thickens, differential buoyancy causes large lithospheric stresses which eventually lead to (plastic) failure in the upper portions of the lithosphere. This plastic zone thickens faster than the lithosphere, so that at some critical time, it reaches the base of the lithosphere. At this point, the effective lithosphere viscosity decreases to that of the underlying mantle, and subduction can occur. We suggest that this is mechanistically consistent with the postulated Venusian tectonic style.