I recall some 'insider info' reported here a year or two ago to the effect that NW flags pax that generate high revenue, and will give a little bump up to high-rev pax that don't quite make the cut on miles/segments. If your tickets are often in the upper fare buckets, that could explain it.

Is it a once-only thing? On one hand, I'd assume if you keep your flying patterns the same, you'd continue to get the same treatment; on the other hand, I would never count on getting anything given to me by an FF program without meeting the published criteria. So, my suggestion is don't rely on it continuing to happen, but if it does, then ^.