The teams they play might have under estimated their talent level. And think it will be a cake walk they don't have to play hard to win just show up and they will win. Or the name on the front of the jersey will scare them and they won't be able to play them because of this. You need to respect who you are playing or you going home early.
They were a #1 team. #1 in their conference. That's how they got into the tournament. The seeding committee put them at 15 because they weren't ranked in the nation and hasn't been seen by many people before. Think of Gonzaga a couple years ago. They were a low seed and unknown. Now they get respect.
That's the really neat thing about the NCAA Tournament...any team (almost) can beat any another team on any given day. While that's not always likely, it can and does happen. That's what sports dreams are made of. Underestimate you opponent at your own risk.