Just did some research on Clete Bakeman's crew (the one for eagles @ dallas game) since I noticed they really let the guys play. My hypothesis is that less DPI and defensive holding calls lead to lower scores.
Week 1: Giants @ Lions (Over)
Week 2: Phins @ Bills (under)
Week 3: Jags @ Chargers (over)
Week 4: Cowboys @ Rams (Over)
Week 5: Didn't officiate
Week 6: Giants @ Eagles (Under)
Week 7: Cards @ Raiders (under)
Week 8: Balt @ Cinci (Over)
Week 9: Saints @ Panthers (under)
Week 10: Falcons @ Bucs (under)
Week 11: Didn't officiate
Week 12: Pack @ Vikings (under)
Week 13: Eagles @ Cowboys (under)
So, this crew so far has had 7 unders and 4 overs, with 6 of their last 7 games going under. Besides the first four games of the year, a game that Clete Bakeman's crew has officiated has only gone under a single time.