He looked surprised that the ball carrier dropped his shoulder and accelerated.
The ball carrier was dead to rights on being tackled in an open field tackle. By lowering his shoulder and accelerating, he was doing what they all do—just trying to get an extra yard or do before going down to the turf. No way he could anticipate that the defender would pause and end up being a sitting duck like that, allowing himself to be struck in the center of mass like a statue.
...or so...