The stations near the coast are recording the motion of the North American plate as it gets pushed to the northeast by the descending Juan de Fuca plate. This only occurs along the coast, where the two plates are locked together. When the increasing pressure of being pushed overcomes the friction between the two plates, the North American plate will snap loose, causing an earthquake.
The stations in the east show much less movement, because in this area the Juan de Fuca plate slides smoothly underneath, and there is little or no build-up of stress.