A rock slide occurs when rocks sit on a sloped plane, and added weight or water causes them to slide down. Heavy rains weigh on the rocks and dirt, and also make the underlying surface slick. Clay or dirt erosion under rocks adds to the potential for a rock slide.
The domino effect that causes a massive rock slide often begins with one large rock or a small section of rocks sliding down the slope. The momentum of higher-positioned rocks cause other rocks and dirt in their path to move. Rock slides are dangerous to people, and cause problems to buildings and roads in their path.