From 2008 through 2010, Kevin Youkilis of the Boston Red Sox saw 35% of his balls in play end up as grounders. While he hit .393 on balls in play in that time frame, he only managed a .260 average on the grounders. In 2011, his ground ball percentage rose to 41.4%, with a .270 BA on his grounders. Over all this season, when he puts the ball in play, he's hitting .347.
The change in ground balls rates comes from changes in both the pitches Kevin sees, and his approach at the plate. In the three previous seasons, pitchers worked him away, but used the entired vertical part of the strike zone:
In 2011, this shifted down a bit:
Pitchers are using the top of the strike zone less, and are not afraid to go out of the zone on the lower edge.
In the 2008-2010 time period, Kevin liked to swing at inside pitches, but he also used the whole vertical part of the inside of the plate:
Especially at Fenway Park, high pitches could result in fly balls that scrape the Green Monster for hits. This season, he's looking lower:
Pitchers work Kevin lower in 2011. He appears to be looking lower, swinging lower, and hitting more ground balls. That's hurting his batting average overall.