I have studied and reviewed every single MLB Over/Under literally every single day over the past two years, not to mention just about all of them from 2009-2010, and this was definitely one of the more peculiar ones. What was even more mind-boggling was the line movement, going from 8 to 7.5, considering there's no way most of the general public's action would be on the under, considering 90-percent of people unfortunately have no clue who Hisashi Iwakuma is (Successful Japanese pitcher; much better than his current numbers if you've seen him in long-relief action).
That said, Vegas probably figured he was due for a good start, since he's had success outside of the Major Leagues before, and has displayed an ability to be mildly successful in this league. In addition, James Shields was more than due for his trademark excellent home start, with one key variable: It was James Shields T-Shirt Night (He has this every year, in which he designs his own shirt and it's given out to the first 10-20,000). Mentally, especially to a somewhat attention-liking established veteran like Shields, this gives him a bit more of an edge and absolutely makes him more motivated, at least as it pertains to this start, in effort of putting his struggles behind him.
A lot of people around the country definitely fell for this over. Vegas had this one completely mapped out, although anything can still happen with the bullpens, given how historically bad they've been around the league this year.