As you might expect, every aspect of the Phillies' game is poorer in Philly. Despite playing in the hitter-friendly confines of Citizens Bank Park, their home OPS is 47 points lower than their road OPS. They're also the only ones having trouble scoring in Philly. They've been outscored 199-164 at home and their pitchers have a collective 5.37 home ERA, compared to 4.05 on the road.
So what's going on here? In short, it's a total collapse of the lefties. The Phillies are an incredibly lefty-heavy team, both on the mound and at the plate. Nearly all of their stars — Chase Utley(notes), Raul Ibanez(notes), Ryan Howard(notes), Cole Hamels(notes), last year's surprisingly effective Jamie Moyer(notes) and promising rookie Antonio Bastardo(notes) — are southpaws. Though the team's righties have been mediocre to bad no matter where they've played, the Philadelphia lefties hold a massive contrast between their home/road splits.
RH hitters (H/A) — .711/.715
LH hitters .848/.965
RH pitchers 5.28/4.54
LH pitchers 5.46/3.45
Righties have virtually the same OPS at home as on the road and RHP have been similarly mediocre, though they've pitched worse at home — understandably, since The Bank is a hitter's park. But their lefty hitters, inexplicably, are giving up more than 100 points of OPS at home and their lefties' ERAs are a stunning two runs higher at home.
So why are Phillies LHP pitching so badly in Philly? As you might expect, they're giving up more hits, more walks, and more homers, so even though they're striking a few more guys out they're dealing with a lot more baserunners. And their home offense is being hurt by a major lack of luck — though their righties are hitting just as badly on the road as at home, at home they're being killed by a .261 Batting Average on Balls in Play (BABIP), which would hamper their ability to take advantage of the shorter left-field wall. Righthanded pitchers have also had a BABIP 30 points higher on the road than at home, so they've arguably been unlucky too. Their southpaws' BABIP is higher at home, but only 15 points higher.
So it's been a confluence of bad luck, bad home field dimensions, and regression to the mean after winning a World Championship. They probably won't keep playing this badly at home, but their stars are heavily left-handed, and that makes them vulnerable. That weakness is being exploited on a frequent basis by the visiting clubhouse.