It's not just the water, the hill, or gravity.
CA has hot, dry summers also. If the grass isn't burned from the wildfires they have every 2-3 years then the grass doesn't grow a substantial root system due to the lack of water.
Take this 'loose' soil, add some substantial rain, then...viola, someone's house has moved several hundreds of feet.
I find it somewhat strange that people still clasp their hands to the sides of their faces with their mouths wide open (ala Van Gogh's Scream) loudly proclaiming how surprised they are at all the devestation of the has caused.
Come one, it's CA. A natural disaster occours 2 out of every three years. Pick one, wildfires, mudslides, earthquakes.
BTW, leading experts are saying CA is due for another earthquake.