According to forbes.com, about 181 million viewers tune in to March Madness throughout the tournament, over phones, tablets and the television. These nationally recognized games may likely and understandably cause players to crack in clutch, high pressure situations, causing upsets. While veteran athletes may be used to the limelight, many college basketball players, some only 18 or 19 years old, have never been a part of an event even a fraction of the size of March Madness. Most of the pressure would likely be on the team expected to win, and depending on the player or team they could either play better or worse as a result of this pressure.
While high pressure situations are a major part of why upsets occur, tournament structure is an even larger part. More than any other league in the sports world, the NCAA National Championship Tournament sets up the possibility of an upset. First off, March Madness is a single game, always-sudden-death playoff series, which ensures that at any given game, an upset can occur. In leagues such as the MLB or the NBA, playoff games are held in a "series" of best of 5 or 7 games, and these systems practically eliminate the chances of any major upsets.
More than high pressure situations and tournament structure, team size contributes the most to why upsets occur in March Madness. Basketball, unlike football and baseball, is unique by having only 5 players on the court at all times. This team size enables schools to reach success with only 1 or 2 great players, which, with a good recruit, is not too challenging to achieve. I think that it is because of this that most upsets occur, because as soon as a team gets one recruit, more and more start coming, and even though the school may be small, they could have a good basketball team.
More than the tournaments of other popular sports leagues, underdog teams are set up for success in March Madness, due to high stress situations, tournament structure and team size. This combination of variables that cause upsets are what make the tournament special, and ultimately what makes March "mad".