In the case of Aberfan, it was a combination of both human and naturistic events that lead to the disaster. However, not all cases are the same.
It is thought that over use of upland areas for agriculture, tourism etc can signinficantly increase the inducement of Land Slide. It is not unalike the process the of Snow slides or avalanches. Over exploitation of the hill and the increasing pressure upon occupying agencies will continue to persuade over use of upland land.
Although the naturistic effect can never be fully halted, the increasing anthropogenic risk to high land will continue to increase the risk and jeopardy of the hill side.