Natural slopes are generally in equilibrium. The degree of steepness, amount of rainfall, soil thickness, angle of repose of rock debris, and vegetation combine to produce a slope that will tend to stay put until something changes. In many cases something does change that weakens the slope. The changes can be natural, such as a large rockfall sheet that overloads a slope, a run of several unusually wet months or years that saturates and weakens upper rock layers and soils, or stream erosion that undercuts the base of a slope.
Other changes can be induced by humans. The excavation of roadcuts, quarries, or trenches may remove supporting earth and rock from the base of a slope. Conversely, piling up mine tailings, landfills, or earth embankments for building sites or roads may overload a slope. In addition, any of these changes may alter the natural runoff of rain or snowmelt and streams.