Jon Lester of the Boston Red Sox allowed 14 home runs so far this season in 97 1/3 innings. He gave up the same number of homers in 208.0 IP during the 2010 season. What changed?
The majority of home runs off Lester occur on his fastball, so this study will concentrate on that pitch. The first thing to notice is the location of Lester's home runs:
Lester tended to work to the catcher's left hand in 2010, but in 2011 moved to the right hand. Home runs came up and in in 2010 because that's where Lester threw the ball. That's no longer true in 2011.
Batters are also teeing off on fastballs that don't stay up as much:
Although not shown here, the overall movement of Lester's fastballs hasn't changed. Hitters are able to key on the ones that don't stay up as much, however. "Rising" fastballs are difficult to hit for home runs, since batters get under them too much. In 2011, batters are hitting the ones that come in flat.
That may be in part due to a drop in velocity. In 2010, Jon averaged 93.5 MPH on his fastball, and 94.2 MPH on the home runs he allowed. In 2011, his average fastball comes in at 92.5 MPH, 92.6 on home runs. A batter has an extra tick to recognize the ball and line it up properly.
The biggest factor appears to be Lester's overall pitch location. Working high to the catcher's glove hand gave batters trouble. Now that he's working more over the center of the plate opponents can make better contact, and they can drive the ball out of the park more often.