There are several reasons for landslides after a heavy rainfall.
1. When the dirt has absorbed enough rain, it becomes mud. Mud is much more mobile than dirt. All mud needs to move is an incline.
2. Water is an amazingly heavy substance. When you add all that additional weight to a mobile subtstance on an incline - you get a landslide.
3. The last factor is that there haven't been any precautions taken (trees, dams, braces, etc) to prevent the mud from becoming mobile or to stop it if it starts.