Tournaments are like time sped up. In the intensity of a few short days, teams can go through changes that might take years in normal circumstances. Think of Brazil in last year's World Cup, for example: they went into the competition on a run of nine straight victories only to unravel under the pressure of playing at home.
The reverse can also happen. A team can suddenly come together during the course of the tournament and end up doing things that it never believed possible before a ball had been kicked. Perhaps Paraguay in the 2015 Copa America are an example of this phenomenon.