I disgree with the lot of you and I shall defend my position by breaking it down like the Zapruder film.
In this first frame, Quan is headed toward the sideline. The defender is still about 3 yards away but is closing fast. Note Johnson's position relative to the white line that extends out from the boundary. His right foot is almost even with the white line. His feet are shoulder width apart, his hands are in his pockets, and he is standing completely upright. At this point there is no reason for him to think he is going to get hit.
In this second frame, Quan clearly is completely out of bounds. The defender has not yet made contact. Notice Johnson's feet. His right foot is still in the exact same place but, sensing that there may be a collision, his left foot has started to move back and to the left. Away from the potential point of contact. Back...and to the left. He is still standing completely upright with his hands still in his pockets.
In this third frame, the defender has now hit Quan out of bounds (should have been a 15-yard penalty), altering Quan's momentum directly toward Johnson. Note that Johnson has now moved his right foot backward in an attempt to avoid the collision. He appears to be bracing to protect himself from inevitable collision.
This final frame shows the actual impact. Note that Johnson's leg is not extended, as Tom Paris suggested. In fact, it is still moving backward, away from the collision. His shoulder is not lowered. He is simply bracing in a defensive posture to protect himself from the impact.
In less than a second, Johnson went from having no reason to believe he was going to be involved in a collision to being involved in a collision. He did nothing aggressive or malicious. He simply protected himself.
It should have been a 15-yard penalty on the player for a late hit out of bounds, but Johnson did nothing wrong.