The term mudslide works too, except that the mud describes the bottom, deadly part more than the upper part of the event.
In the Stilliguamish case, the hillside is made of loose sandy material laid down by glacial lakes less than 16,000 years ago, according to Washington State geologist Dan McShane of Stratum Group who writes a blog called Reading the Washington Landscape. Since then the Stilliguamish River has cut down into that loose material, creating hillsides of the sandy material that have a history of collapsing.
Those hillsides were apparently saturated with water when the one collapsed on Saturday. As anyone knows who has tried to build a sand castle, too much water causes the sand to flow and not stand up. In microscopic terms, it's the grain-to-grain friction that holds sandy slopes together. When water fills the spaces between the grains and pushes the grains apart, the sand, and in this case an entire hillside, collapses.
The upper part of the Stilliguamish/Oso landslide appears to have slumped, and rotated -- like a person sliding down off a chair. The lower portions were even wetter and apparently behaved more like liquid -- a mudflow.