Notice how the rocks, gravel, soil, snow, boulders and other debris flows like water, even though it is all solid material.
That is exactly what a volcanic lahar looks like, only smaller. This was only a rockslide/avalanche but if Mt. Ranier ever erupts, it will trigger a lahar that will flush the Puget Sound like a giant toilet, right out into the Pacific Ocean. So many people live in the likely path of such a lahar that if a sudden eruption of Ranier did occur, evacuation would be nearly impossible and the death toll would be staggering.
Recent Mount Rainier Earthquakes