My assumption is that they have migrated this way from Canada, and possibly from Idaho, Montana, and Wyoming. The Yellowstone wolves that were reintroduced in 1995 fanned out, and it is possible that as the Yellowstone wolves moves they displaced non-related packs westward towards Washington and Idaho.

WDFW's take on gray wolf recovery and why we've seen an increase.

http://wdfw.wa.gov/conservation/gray_wolf/