It is far more likely that these observations were related to several powerful
hurricanes that made landfall at the same time that the plate motion "reversed".
This change in motion was not just observed in the northwest, but throughout
the west where instruments measure such movements. The article gives a
provincial view, but at the time that these events were occurring we were wondering
in various forum whether the reversal of motion of the North American plate was
related to the landfall of the hurricanes which now, after more data examination
seems probable. There have been several threads dealing with this odd reversal
of motion and the hurricanes associated with it - they can be found in the archives,
but I'm not sure where.
What appears to be true, however is that the change in motions started at
hurricane landfall and involved the whole North American plate, not just the
small section examined in this study.