NCAA guidelines:
3. Step three is to fill in the bracket so that first-round conference matchups are avoided, unless it corrupts the integrity of the bracket. If five or more teams from one conference are selected to the championship, then the integrity of the bracket will be protected (i.e., maintaining the pairing process according to seed will take priority over avoidance of first-round conference matchups). To complete each regional, the committee assigns one team from each of the remaining seeded groups so there is a No. 1, No. 2, No. 3 and No. 4 seed at each regional site.