Final answer:
The landslides in Laguna Niguel, CA in 1998 were caused by heavy rainfall leading to saturation of soil, which weakened the slope materials resulting in a landslide.
Step-by-step explanation:
The landslides in Laguna Niguel, CA in 1998 were primarily caused by heavy rainfall and the saturation of soil, which weakened the slope materials and led to the ground movement. Other factors that contribute to landslides include erosion by rivers, glaciers, or ocean waves, creating over-steepened slopes, and stresses from earthquakes can also trigger landslides.
Adding to this, the weight from the accumulation of rain or snow, as well as man-made structures can overstress these weakened slopes, leading to failure. In the Laguna Niguel incident, human factors such as construction may have also played a role, but the critical trigger was the saturation of the hillside due to heavy rains.