So many factors at work.
First, I think teams do have letdowns. Definitely on the preparation side, which in sports played at this level makes a huge difference. Teams "play down" to the competition all the time. Better preparation, better leadership, better whatever could fix this, but there's a reason that there's more to coaching at this level than just recruiting.
Second, I think teams get behind early and fall apart. This happens at all levels of sport (and life, frankly). This is more likely to happen if the team already feels like it isn't getting its due, which is part of the letdown dynamic. To Cincy's point about teams losing when they are favored, it doesn't mean they shouldn't have been favored, it just means they lost. Again, Purdue shouldn't have crushed Ohio State--a 1-loss, Rose Bowl winner--but it did. The ball starts rolling and sometimes it doesn't stop.
This goes to the "they showed up to play" issue. Yes, when the lights go on, they run out of the tunnel, and they line up for the first play, they are jacked and ready to go. But when they are all of the sudden--unexpectedly--down two scores, they remember the feeling of loss from the last game, they don't get a call they really wanted, and they have another turnover, players--and teams--absolutely do fall apart. Not all teams, not all the time, but this is a team dynamic. Good leadership prevents that--so Georgia and Washington fought back into games that looked too far gone (they still lost, but they went down swinging); Miami imploded against Wisconsin. Was 7-5 Wisconsin that much better? A team that lost at home to BYU and was kicked around at home by Minnesota? No.
Third, many of these teams were overrated going into that game they lost. The 2006 Big Ten teams appeared to be overrated. If I remember correctly, USC smoked Michigan and Florida smoked Ohio State in the MNC game. Maybe neither deserved the 1 or 2 ranking they had when they played each other. Wisconsin had 1 loss that season and took all it could muster to beat a one-dimensional, good-but-not-great Arkansas team in the Capital One Bowl. Maybe the Big Ten just wasn't that big that year, which inflated both Michigan and OSU's rankings. Maybe the Pac-10 (USC, Oregon State, and UCLA) and the SEC (Florida, LSU, and Auburn) were. Kansas State and UCLA in 1998 are other good examples. They lost those games because they weren't elite teams, despite their rankings.
Fourth, as already noted, these are often among the best teams each of these teams played all season, so it isn't surprising that they are tough outs.
And finally, some teams are better week-to-week in the regular season, others have coaching staffs that are really good at preparing for bowl games. Bret Bielema fielded better teams week-to-week; Barry Alvarez coached them up for bowl games. They aren't the same.