[TRWSHSHLX]

There are a few components to upsets in regionals. Since the qualifying rounds schedule is totally random sometimes a good team might be put against another good team but with better alliance partner and lose those matches. That'd cause a good team to be ranked lower than they really should.
Secondly there are always the problems of debugging. Like some posts mentioned, some teams do not have all the bugs worked out until the elimination rounds. Those teams are usually overlooked but if picked, it can be a huge factor.
And lastly, the ranking system that FIRST uses to rank the teams are purely based on offense. The GDC encourages offense and an offensive robot or a team that's always with an offensive robot (not likely) will get really high ranked. However, there's no mention of defense in the ranking system what so ever. If a robot is a mostly defensive robot, it will most likely rank relatively low and sometimes they're overlooked but can be another major game changer because in elimination rounds, it's the point difference between alliances that matters, not as much as you can score, like in qualifying rounds.
All in all, a successful alliance will have both offense and defense. And the alliance that can execute always wins.