This could explain why he's lowering his head & not doing as many jump cuts.
''Johnson has been implored by running backs coach Kirby Wilson to run more downhill and not bounce outside as much. The goal, Johnson said, is to turn 1-yard runs into 3-yard gains. While he did it more Sunday in Kansas City, it wasn't easy. One of Johnson's favorite moves is the jump-cut. Lowering his shoulder and bulldozing a defender isn't natural for him, he added. He'd prefer to create with cuts, moves and jukes.''
http://www.espn.com/blog/arizona-ca...is-is-the-david-johnson-weve-been-waiting-for