cappin', i have a theory on that. i haven't had a chance to do the research to back it up yet, but.... in the early bowl games, there is generally a lack of motivation, which i believe will more often affect the defenses. i would expect this to be even more prevelant in games involving teams from the major conferences. then, once you get to new years day, the teams should care a lot more and i'd expect the games to start going under.
i just took a quick look at last year. prior to 12/31, the over was 10-6. new years eve and after, the over was 6-6. seems to bear out what i was thinking.