A friend asked me on FB what my impression was - I'm not an engineering geologist or geotech, but I work with them (and am therefore qualified to play one on TV).
By all accounts this is a 'deep seated' slide - they are not uncommon up here: http://www.ecy.wa.gov/programs/sea/landslides/about/deep.html
There was a similar slide on Whidbey Island almost exactly one year ago: http://washingtondnr.wordpress.com/2013/03/27/whidbey-island-coupeville-landslide/
And along the Nooksack River in February: http://washingtonlandscape.blogspot.com/2014/03/more-on-clay-banksnooksack-landslide.html
The current one made the national news due to its size and the tragic loss of life.
These slides are large and very difficult to predict because they involve the activation of blocks of earth along deep slide planes. Combine with both the weight and lubrication of water saturation, and they can release. They occur along bluffs, especially those with water erosion at the base - beaches, or in this case, a river. Everything wants to go to its lowest state, its most stable. Occasionally that means a huge block of soil and rock rotates from a higher elevation to a lower. It is the landscape seeking its angle of repose.
The ironic thing is that this slump of soil, rocks and treee will possibly have a higher death toll than Mt. St. Helens. At least she gave us a bit of warning to step out of the way.
"Civilization exists by geological consent, subject to change without notice."
- Will Durant