Vernon Wells of the Los Angeles Angels of Anaheim own the lowest OBP in the major leagues among hitters with at least 350 plate appearances. Looking over various numbers for Vernon over the last four seasons, you can see the deterioration:
In the three previous seasons, Wells did not post a great OBP, but it wasn't terrible, either. They varied from a bit above average to a bit below average. Both his strikeout and walk rates were rising slowing over the three previous seasons, but in 2011 the K rate sky rocketed while the walk rate fell in half. That's not surprising given that his strike and ball rates went in the same direction.
More strikeouts mean fewer balls in play, so Wells would need more of those finding holes for hits to make up for the Ks. Instead, his batting average on balls in play (BABIP) dropped 65 points. Vernon suffers the double whammy, fewer balls in play, and a much lower percentage going for hits.
Why? His approach at the plate is off. He's swinging at more pitches, and missing on more of those swings. Part of that comes from chasing more balls out of the strike zone. When he does make contact, few of those balls result in line drives, the type of ball in play most likely to result in a hit.
Often times, a low BABIP is indicative of bad luck. Given all the statistics that deteriorated for Wells this year, this looks more like a drop in skill level than a fluke bad season.