Here's why:

* Saturation: Heavy rainfall saturates the ground, making the soil heavier and more prone to sliding.

* Slope instability: Rainfall can weaken the stability of slopes, especially those that are already steep or have been disturbed by human activity.

* Erosion: Rainfall can erode the soil, creating channels and undermining the stability of slopes.

While heavy rainfall is the primary trigger, other factors contribute to landslides in California:

* Wildfires: Burned areas are particularly susceptible to landslides because the fire destroys vegetation that helps hold the soil in place.

* Earthquakes: Earthquakes can shake loose soil and rock, increasing the risk of landslides.

* Human activity: Development, deforestation, and improper grading can all contribute to landslide risk.

It's important to remember that landslides are a natural process, but they can be exacerbated by human activities.