The term 'bracket buster' in basketball refers to a lower-ranked team that unexpectedly wins against a highly-ranked team, thus busting the bracket predictions of numerous fans who had the higher-seeded team going deep into the NCAA tournament. One of the most famous bracket busters was Middle Tennessee State in 2016, who pulled off a shocking upset against Michigan State in the first round. Michigan State was a 2 seed and a popular pick as champion across brackets. Middle Tennessee State pulled off the win as a 15 seed, leaving the overwhelming majority of brackets busted.
A bracket buster does not have to involve the lowest seeded team defeating the highest ranked team, although this is a textbook example of a bracket buster since very few people predict such a dramatic upset. Ultimately, 'bracket busters' are merely just instances where a team that a person picked to go far in the tournament loses in the earlier rounds.
Teams are assigned 'seeds' based on their regular season records and strength of schedule. For example a team that amasses an impressive win-loss record while playing in one of the tougher conferences any year is likely to be a high seed, while a team that posts a decent record in a mediocre conference is likely to be a lower seed. Seeding is indicated by a number assigned to each team before the tournament begins, with the highest seeded teams being in the 1-4 range and the lower seeded teams being anywhere from 10-16. The higher the seed, the lower the number that is given to the team.
In the early rounds of the March Madness Tournament, the highest seeded teams (seeds 1-4) are matched up against the lower seeded teams that are perceived to be far worse by the committee that organizes the tournament. Many people automatically assume that a number 1 seed will easily defeat a 16 seed, or a number 2 seed will dominate a 15 seed. While this assumption is usually correct basketball is a sport in which anything can happen, and there have also been some instances of the much lower seeded teams shocking the world and defeating a powerhouse basketball program.
March Madness brackets follow a very specific scoring format, in which participants receive points in increasing intervals for every correct pick as the tournament progresses. The term 'bracket buster' comes from the harsh reality in which a team that was picked to go far in the tournament loses to a seemingly unworthy opponent in the first or second round. When this happens, participants automatically forfeit points for every game that they picked the losing team to win. This severely diminishes their chances of being crowned champion of the bracket pool.