Bobby Abreu's batting average and OBP dropped about 40 points in 2010. Part of that was due to his walking less and striking out more. When those two statistics move like that, it would seem the batter was getting more aggressive at the plate, maybe swinging at more bad pitches. The opposite seemed to be the case with Bobby, however.
The following heat maps show Abreu's swing rates during the 2009 and 2010 regular seasons.
Note that Bobby has three distinct swing zones, and that he's not afraid to chase an outside pitch.
In 2010, Abreu swung mostly at pitches up and in, and did a great job of taking pitches just off the outside corner. So why, with better selectivity did his walks go down and his strikeouts go up? The following two heat charts show his strike rate during these two seasons.
Even through Bobby stopped swinging at outside pitches, he was still getting as many strikes off the outside edge of the plate. I suspect this came from two umpire biases. Batter often define their own strike zones, especially selective ones like Abreu. Because he swung at outside pitches in 2009, those became part of his strike zone.
The second I'll call the Jack Clark effect. At the end of his career, Clark could no longer hit very well, but retained his great eye for the strike zone. In 320 plate appearances that season, he walked 56 times and struck out 87 for a .210/.350/.311 slash line. As the season went on the umps realized Jack was taking more and more, and expanded the strike zone to encourage him to swing.
When Jack was willing to swing at strikes, the umps gave him the benefit of the doubt on border line pitches. When Jack was no longer willing to swing at strikes, he lost the umpires and the border line pitches started to go to the pitcher. That seems to be the story with Bobby. He no longer swings at pitches clearly over the outside part of the plate, so the umpires have no reason to give him the benefit of the doubt on selectivity. I suspect that like Jack Clark, age finally caught up to Bobby.