Interesting call for sure. I read the actual rule this morning and there is a provision for a defender "turning his back" to avoid being legally blocked. I think that's a reasonable interpretation of what happened. The defender was running parallel to the LOS while Saffold was running straight upfield. He absolutely saw Saffold coming. As they converged, Gurley cut back and the defender tried to plant and twist back toward the play, essentially "turning his back" as Saffold hit him.

In hindsight, Saffold's best bet would have been to do nothing, or maybe just get in the way a little and not extend his arms.