You've seen it happen. A golfer is playing great, everything is flowing and they seem unstoppable.
Then something happens. A shot goes bad or a gust of wind costs them a stroke (that happened to Harrington at the 2009 Masters). They can lose their rhythm, confidence or focus. Shots go bad, they can choke, get the yips, get more and more stressed and they end up in a vicious circle of frustration. At that point, they've lost the mental game.