Anthropomorphic (human) effects can accelerate or even cause landslides to occur in locations that were stable.  Not understanding the mechanics of landslides can have disastrous consequences.  The middle left diagram shows a slope that has been over-steepened during construction.  The slope will then slide until it is stable again.  The diagram on the middle right side shows a case in which additional weight is placed above a slope.  That can cause the slope to fail if it was close to failing to begin with.  The lower left diagram shows how a stable slope can be made unstable by saturating it with water.  The final, lower right, diagram shows a case in which the slope is made to fail by removing support at its base.  This type of failure happens when terraces are placed at the foot of slopes to act as drainage ditches or extra shoulder room on a road.