[sritchie]

One way is wind loading... snow falling in a storm gets blown around and piles up on a slope that's already loaded. Another type - a wet slab avalanche - triggers when the sun melts snow and causes water to flow between the various layers in the snowpack, causing two layers to slide.

The goal of the bomb is to thump the snow hard enough that if any weak layers of snow exist below a hard slab of solid snow, the slab will crack, slamming air through the weak layer, breaking the support of the rest of the slab and causing a slide.

Naturally occurring avalanches usually need some obvious change of force on the snowpack, or some reason why a weak layer becomes weaker. I'd guess that a sonic boom, or definitely a scream, couldn't do enough in almost any situation to tip the balance.