With the drought, the ground is no longer stable. The tree roots have shrunk and moved to the topsoil, so there is no structure holding the dirt together. When the rains come, enbankments and cliffsides just collapse.
A good reminder to think about where you are parking and in what conditions. This type of stuff is going to keep happening.
As,an additional caution: be careful about parking under/near redwoods. Redwoods have shallower root systems than other trees. In natural settings, they counter this by interconnecting their roots with the other nearby redwoods. This creates a strong support system for the tall trees, keeping them from toppling. However, when the trees are planted by man, they tend to be planted singly - with only one or two redwoods in close contact. This means that the trees do not have the support system at their roots to counter their shallowness. Add the drought and erosion of the topsoil, and these trees are far more likely to topple in a storm.
