They're caused by when you have wind shear -- that is, layers of air moving in different directions. As those layers interact with clouds, you can get turbulence that causes these impressive wave-like formations to occur. (A much more in-depth physical description can be found on the Univ. of Illinois Urbana-Champaign website
The cloud formations aren't foreign to the Pacific Northwest -- we've had a few sightings over the past few years.
They're pretty to look at on the ground, but for pilots, they signal an area to avoid as it can be a source of severe turbulence.