Nate McLouth saw a huge falloff in his OBP in 2010. He came into the season with a .342 career OBP, but finished the year with a poor .298 mark. The falloff was almost completely due to his hitting; he still walked a decent amount during the season.
What happened? Note that during the 2008-2009 seasons McLouth showed great plate coverage. He could knock out hits inside, outside, as well as down the middle:
Nate tended to hit well inside, so pitchers worked him away:
In 2010, cold spots appeared in his strike zone:
That last graph is interesting for another reason. The software that produces it can show a time lapse image. When you watch it change over the 2010 season, the distribution starts out very much like the 2008-2009 image above. As it becomes apparent that Nate has a hole on near the outside edge of the plate, the league brings the pitches in to challenge him there, rather than trying to get him to swing at ball off the outside edge. The league noticed the problem and adjusted to the batter's weakness, and McLouth did not find a way to adjust back.