As you know, gravity is always pulling on us and the rocks around us towards the center of earth. As rocks weather and weaken, they eventually break and cause mass land movements.

There are a variety of different types of mass movements from simple rock falls to mudflows. You can read more about each kind at:
this site.

Though we did have a tragic landslide in Southern California recently (La Conchita), most mass movements in the USA do not kill people. They do, however, cause significant damage to houses and roads that can be very expensive. If we plan better as a society, we can prevent many of the more major expenses.