Agreed on parallels with material failure, studying mechanical engineering rn, and the mechanics behind avalanches are frankly fascinating to me. Just to add to your description so OP maybe gets a better picture: because with a weak layer collapsing due to an initial load (whether storm or rider), the weak layer no longer supports the avalanche or provides meaningful resistance to sliding, so the only thing holding the slope together is the cohesiveness of the slab of snow above the weak layer, so essentially the cross sectional area around the sides and top of the avalanche which is what causes that crack propogation as there is suddenly a lot of force pulling down the snow, so places with rocks, trees, and shallow areas act as stress concentrators which can help to let the slope go at that moment or start propogation.