Experimental evidence shows that snow is a pressure-sensitive, dilatant, strain-softening material in slow, constant-rate shear deformation. When strain-softening initiates in a weak layer underneath a snow slab, avalanche release is hypothesized to be possible with or without loading. Specifically, two cases are discussed : (i) a shear-crack-like disturbance can initiate by formation of a slip surface in the weak layer and traverse the layer by a self-propagating progressive failure with or perhaps without loading; (ii) a self- propagating shear instability can develop when a region of the weak layer is driven past peak shear strength by loading. These cases represent the extremes in weak-layer deformations under which strain-softening failures might precipitate avalanche release. For these cases the associated fracture sequences, fracture geometry, time scale of release, and temperature- related effects are consistent with the known facts of dry-slab avalanche release.