An avalanche usually happens when a layer of snow collapses, and begins to slide downhill. They can be caused by any one of four factors.
This is; a steep slope, snow cover, a weak layer in the snow or a separate trigger.
A trigger could be caused by fresh snowfall adding a new layer to an already existing snowpack. If it becomes overloaded, a slide may start.
Earthquakes can also set off avalanches, along with much smaller vibrations.