Your park penalizes righties. You have also faced tons of lefties this year(Usually 3-1 in at bats for righties vs lefties, but your closer to 2-1). Your in the bottom 5 vs lefties. Throw in some underperformance, and there you are.

Some unsolicited advice. Since you are only a few games out from the wild card, pick up a couple right handed bashers vs lefties to platoon. Also pick up another reliever. This could propel you to a wild card(most teams would hate to fact Schmidt - Johnson in the playoffs).