In the FBD, friction from the bed surface is creating a force in the opposite direction of motion. 
The Fc force is the compression force from the snow down slope of the slab pushing against it.
The force from the weight of the slab and skier only matters in the x direction concerning avalanches. A steeper slope angle increases the force exerted in the x direction.

Slab avalanches are are caused when a weak layer supporting a dense slab collapses from excess forces applied to it. The dense slab then begins to slide on an underlying bed surface. This can happen naturally from added weight of storm snow, wind transported snow, and sun affected snow. 

Humans also can apply force to the slab causing the weak layer to collapse. Most human caused avalanche fatalities occur during skiing/snowboarding and snowmachining. 

Because the dynamics of the forces applied to a slope from a snowmachine track are difficult to measure, I'm going to create a free body diagram of a skier causing a deep slab avalanche. 

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