This is a simple one. When a player has a good lead or is playing a seemingly weaker player, they often play to "not lose." This means lowering their own level of play, since they are no longer doing what they normally do. They are literally abandoning whatever gave them that lead or made them the better player, and then wonder why their level drops. This probably leads to more chokes and upsets than any other single cause.
Instead, no matter the score or the opponent, play your game. Think tactically about what you need to do to win the next point, and the point after, and so on, using your game and what worked before as your reference. Perhaps put an emphasis on consistency when you have a good lead or are playing a weaker player, but only emphasize it, don't go overboard and turn into a quivering mass of passivity. Play Your Game!!!