They are activated by such things as storms and fires, earthquakes, volcanic eruptions, rock and soil weakened through saturation by snowmelt or heavy rains and excess weight from accumulation of rain or snow, stockpiling of rock or ore, or waste piles. Man-made structures may stress weak slopes to failure creating a landslide. Slope materials that become saturated with water may develop a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path. Debris flow across I-70 after Storm King Mountain fire.
Landslides occur in every state.
The Appalachian Mountains, the Rocky Mountains, and the Pacific Coastal Ranges and some parts of Alaska and Hawaii have severe landslide problems. Any area composed of very weak or fractured materials resting on a steep slope can, and probably will, experience landslides. Click on the link here for the USGS webpage. They have a lot of information on the the DeBeque Canyon landslide in Colorado.