Z has been relatively consistent when it comes to babip, and consistently below the league average. This season sticks out like a sore thumb. A lot of babip is luck, but you have to factor in some observational stuff, like how often a pitcher is being squared up by hitters. You can expect a guy who gives up a lot of ropes to have a higher babip than one who gets hitters to beat the ball into the ground a lot. In August, when Z really turned it on, his LD% dropped precipitously, and his gb rate almost doubled. That's not all luck. This year Z has a career high babip, his highest LD% since 2002 and has by far had his most fb heavy gb/fb ratio. If I had to hazard a guess as to why most of this year has been sub-par for Z, I would say it is because he has missed up in the zone more than usual, combined with decreased velocity and a bit of bad luck. the higher LD% would explain the increase in BABIP but not the GB/FB ratio. fly balls are turned into outs more frequently than ground balls. but here's something interesting. this fangraphs article says that in 2007, line drives were hits 73% of the time, ground balls 24% and fly balls 15%. when you look at an extreme ground ball pitcher - say, brandon webb - his expected BABIPs from 2004-08 were .310, .317, .310, .311, .297. his actual BABIPs were .295, .305, .293, .294, .297. i think there's enough information to say that webb has not simply been lucky his entire career. one factor could be infield defense, but i note the same trend with guys like derek lowe, aaron cook, chien-ming wang when he was healthy - the extreme GB guys post BABIP's better than their xBABIP. the explanation i can come up with is that all ground balls are not created equal, and their style of pitching is such that balls are grounded more weakly than against GB-FB neutral pitchers and fly ball pitchers. interestingly enough, in derek lowe's two worst years from 2002-09 (2004 and 2009), his LD% did not change greatly but his FB% went up (and hence his GB% went down). using the xBABIP formula, one would expect his BABIP to be lower in those years where he gave up more fly balls, but he actually had very poor BABIPs in those years. so perhaps this suggests that in 2004 and 2009, lowe was either missing his spots more or not getting as much sink on the ball, so the ground balls being put into play were, on average, harder ground balls than the ones hit in other years. one last semi-related note. i also came across this article on fangraphs that compared a fly ball-heavy pitcher (barry zito) to a ground ball-heavy pitcher (webb). what the author found was that while webb pitched down in the zone a bit more frequently than zito, webb's expected ground ball rate was just 48% versus 44% for zito. yet webb's actual rate was 66% versus 39% for zito. so the main reason that pitchers have a strong tendency in one type of batted ball is not because they consistently pound a certain part of the zone, but because of the way that they pitch. i always assumed that ground ball pitchers just pitched around the knees most of the time, but that does not appear to be the case.