A landslide is a geological event, in which the top layer of earth on the side of a hill or mountain becomes loose and moves downwards. Although gravity is the main force behind a landslide the side of a hill or mountain can become unstable due to many factors:
sinking groundwater
loss of vegetation
movement of glaciers
heavy rain
earthquakes
volcanic eruptions
erosion by rivers and waves
All of these events can cause parts of a slope to break off and move.
Landslides can also be caused by humans through
deforestation
more and more traffic
construction work
changing of the landscape due to mining
overgrazing
building new settlements