Important factors concerning the new snow layer include its bond with the old snow surface and density changes which occur within the new snow as the storm progresses. Crusts and slabs are examples of surfaces which new snow sometimes bonds poorly with. New snow density typically decreases as a storm progresses. In those cases where the density increases or fluctuates widely heavier snow can be deposited on top of lighter snow, leading to instability within the new snow.
http://www.snowman-jim.org/papers/rambler.html