I think its just other teams catching up to the Big 12 offenses. I think Saban even said as much after we beat them in the Sugar a few yrs ago. Something like he was going to look into having that type of offense because he saw it was the direction where College Football was going. The ground and pound just wasn't getting it done anymore.
If you look at all the teams that are running high powered offenses now most if not all have defenses that are not on par with what they usually field. Is it because of what OU suffers from, heavy recruiting on the offensive side of the ball? I really don't know.