Rule 13-2: Improving Lie, Area of Intended Stance or Swing or Line of Play

While playing in a tournament, one of my fellow competitors played their approach shot and pulled it left of the green. While surveying his next shot, he walked through a bunker that lay between his ball and the hole. On his way back to the ball, the player raked his footsteps. After he played the shot and finished the hole, the rules official assessed the player with a two-stroke penalty. Can you please explain this situation?