High amounts of rainfall are thought to have been one of the causes of the Gros Ventre slide. The water can act as a lubricant, causing already unstable layers of earth to succumb to the forces of gravity and begin moving downhill.
This is the second factor that is thought to have brought about the Gros Ventre slide. Earthquakes can shake up and disturb layers of soil, causing them to become unstable and move downhill. If you add high amounts of precipitation to the mix, you can get an effect known as liquefaction.
Liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to ground shaking. If this occurs beneath buildings or structures, it can cause the ground to lose its firmness and the buildings sink. In the case of a landslide, it can cause upper layers of earth and debris to become unstable and through the forces of gravity move downhill.
Recent Wildfires on Steep Slopes
When trees and other plant life are destroyed following a wildfire, this can destabilize the soil on steep slopes. No longer having protection from rainfall or the root systems for stability, these bare hillsides can more readily form landslides.
Repeated Freezing & Thawing
Repeated freezing and thawing of the ground can act like a pry-bar between layers of soil. During periods of thaw, water percolates down into the ground, then when it freezes again, that water forms ice. When water freezes, it expands and can force layers of soil apart causing landslides.
Volcanic Eruptions or Activity
Volcanic eruptions, either through the shaking of the ground or lava flows, can cause large amounts of debris to move downhill. Clicking on the map to the left, you will see the outline of the Yellowstone Caldera, which is one of the largest and most active calderas in the world (USGS). A caldera is essentially a collapsed volcano, or a volcanic basin.
Human Modification to Slopes
This boils down to people building where they shouldn't be building. Increases in erosion due to development on steep hillsides or building in the path of susceptible areas significantly increase the risk of landslides.