When snow piles up, it piles up either directly on the ground or in layers that form over time. When the snow builds up on a slope that is about 30 degrees or steeper and it becomes deep enough for something to cause it to slide, then an avalanche can occur.
Sluffs are small avalanches that frequently occur. But when the steep snowfield is a mile or more wide, the slide can involve a mass of millions of tons of snow, leading to great danger toward anything or anyone that lies in its path.
According to Edward R. LaChapelle, who is the Avalanche Hazard Forecaster at Wasatch National Forest, Utah under the U.S. Department of Agriculture. Most times, the avalanches occur on well known and established pathways, but this does not rule out unusual or exceptional conditions that cause surprising snow slides that will catch people unprepared and unaware until it is too late.
According to the National Snow and Ice Data Center, the major factors that are involved with avalanches include,
"...weather, temperature, slope steepness, slope orientation (whether the slope is facing north or south), wind direction, terrain, vegetation, and general snowpack conditions."
The general snowpack conditions and related slides are called loose snow avalanches, and slab avalanches. Each type is sub classified by the following qualities of the snow: dry, damp or wet. Then the sub classes are determined by how the avalanches occur: slides either originate in the surface layers or slabs, or the whole mass can slide.
The motion can be on the ground, as powder creates a powerful gas like air mass, or a mix of ground and air motion.
Most major avalanches occur on a slope of 30 to 50 degrees (where snow is more likely to accumulate to significant depth).
Finally the most major avalanches happen and in temperatures between 25 to 30 degrees Fahrenheit.
The triggers for major avalanches come from various stresses, weather conditions and pressures:
Internal changes to the snow pack and other stress will cause the snow to settle. When that snow is on a steep incline, gravity can cause it to slide. The slide causes more internal deformation, which aggravates the process. This happens when different areas of the snowfield have different temperatures, types of snow, depths, degrees of incline and overall profile. Differences in ground cover are also a factor.
The pressure of one zone against another can create tensions and stress fractures, leading to slab avalanches.
An overload of new snow, falling cornices, and snow falling from trees can trigger an avalanche.
Sunballs and snow wheels that are formations of snow that form and roll downhill after heavy sun, triggering slides.
Intentional and accidental triggers cause avalanches. Skiers and hikers trigger the unintentional ones, while forest management may trigger intentional avalanches with explosives or mechanical devices to control the hazards.
Sometimes, the metamorphism and stress development will resolve itself and no avalanche will happen.
The National Snow and Ice Data Center has a comprehensive website that answers many questions about avalanches and what to look out for when in the snowy environments.