To a certain extent people are learning to pitch to him. They throw pretty bad pitches early, and if he happens to swing then they can attack him and get him to K (29 K/79 AB - absurd). On the other hand, if he takes the first few pitches then he gets a walk (26BB). They just aren't giving him pitches to hit, and he's struggling to adjust.
I was really big on the Ryan Howard bust wagon earlier this year and still am. A guy who K's as often as Adam Dunn, even if he hits 15 more homers, should not have an average anywhere near .300.