Unbelievable. My heart sinks to see these pictures.
I always thought that if the country side had a lot of vegetation that slides like this wouldn't occur. Guess I was wrong.
My heart and prayers go out to the people affected.
Vegetation is good BUT this had been made into a residential area and the excavation of roads, homesites and etc. effectively cut apart the naturally stabilizing mass of vegetation. Just one small weak spot can let go and there will be a cascading effect, not only from the force of gravity, but the effect of vibration which makes solid earth behave as though it was a liquid. It is a phenomenon which is almost unimaginable, and you aren't going to prevent it with the usual treatment of wire mesh and retaining walls or berms of rip rap laid on the surface, as solid as they may seem (earth just flows under and around them like water).
Without massive support structures built deep into the ground (to, and anchored in solid rock), any hillside is prone to slide if it is disturbed enough and/or it collects enough moisture.
Slide events this big happen daily in the Northwest during the rainy season, just not usually in suburban residential enclaves such as this. But, particularly as the Portland and Seattle areas have expanded in population and the semi-rural hills have become the new suburbia, we will see them more often.