There can be an upset in any type of sport. For example, in football, an upset is when the team in last place in the standings beats the leader of the championship; in tennis, it's when a player from the second hundred of the rating beats a player from the top 10. As for boxing, The Ring magazine even gives an award for Best Upset of the Year, and UFC regularly publishes the year's best upsets.
Where did the term "upset" come from?
According to legend, the term was first used on August 13, 1919 at the Saratoga Racetrack after the seventh run of the Sanford Memorial Stakes.
The participants in the seventh race included Upset and Man o' War, who had won the last six races. At the end of the race, Upset won, and Man o' War suffered the only defeat in her career. Later, the horses raced again six more times, and Upset didn't win any of them.
This race was real, but the term actually appeared in print earlier. For example, lexicographer researcher George Thompson discovered the term "upset" being used as early as 1877, a full 42 years before the Upset race.
How to predict an upset?
Study the stats: in matches with unequal opponents, the favourite might be in name only. For example, a championship winning club has been playing poorly recently, while its weaker opponent has instead been performing at a higher level and is on a winning streak. In this case, the team with the championship title will still be considered the favourite because it has a stronger team and bigger name. A match like this may end in an upset, although in reality, the teams are almost equal.
What's an upset in boxing and MMA?
Combat sports are different than other sports because they have more upsets. This is due to the fact that a fighter's status often depends on the number of their titles and fights. For example, in a match between a veteran and new fighter, bookmakers and fans by default consider the more titled player the favourite.
The number of upsets also depends on fighting style. In boxing, upsets are frequent in the middle and heavy weight classes. In these fights, one solid blow is enough to win the fight by a knockout. If both fighters like to stay open, then an upset can happen regardless of boxer status.
Upsets in combat sports are also due to fighters being over-hyped, which is especially noticeable in MMA. For example, Conor McGregor, due to his media presence, has been the favourite for the past five years, regardless of the real strength match. Even in his most recent fights, when he rarely fought and was out of shape, the odds for him to win were rarely over 1.50.
One particularly memorable upset was the 2001 boxing match between Lennox Lewis and Hasim Rahman. Lewis had an undefeated streak of 14 fights and defended his world title 9 times, while Rahman's last 7 fights ended in 5 wins and 2 losses. Lewis underestimated his opponent and focused on his role in the movie Ocean's Eleven instead, only starting to prepare for the match a month in advance. Lewis led the match by points, but in the 5th round, Rahman pulled ahead and KOed his opponent with a hook to the jaw.