an improbable and unexpected victory; "the biggest upset since David beat Goliath"

cause to lose one's composure

defeat suddenly and unexpectedly; "The foreign team upset the local team"

used of an unexpected defeat of a team favored to win; "the Bills' upset victory over the Houston Oilers"

when a higher-seeded (better) team loses to a lower-seeded (inferior) one.

Upset is a term used when referring to a competition, frequently in electoral politics or sports. When an upset occurs, the party popularly expected to win (the favorite) is defeated by an underdog the majority expects to lose, defying the conventional wisdom.