So, McClellan gets beat up by the press over the Plame incident and resents both Bush and the press? That might be the reason he knocked them on their war coverage (which is totally bogus, I watched some of the news conferences at the time and remember how brutal the press corps was):

Peter Baker, previously a White House correspondent for The Washington Post and now a writer for The New York Times magazine, said McClellan — despite years of loyalty to Bush — has a deep sense of betrayal over unknowingly conveying misinformation as press secretary.

The book is “not surprising after talking to him,” Baker said. “You got a sense that his perspective had changed. You can’t overestimate how the CIA case [in which former operative Valerie Plame was outed] left him burned … and being pummeled for passing along untrue statements.”
In “What Happened,” McClellan alleges that he was misled by White House aides, including Rove and I. Lewis “Scooter” Libby, into passing along erroneous information to reporters about the CIA leak case.
In a chapter titled “Revelation and Humiliation,” McClellan writes about a particularly tough briefing in July 2005. At that time, Newsweek’s Michael Isikoff had a scoop about conversations between Rove and reporter Matt Cooper, who was then at Time magazine and is currently at Portfolio.
“I could feel something fall out of me into the abyss as each reporter took a turn whacking me,” McClellan writes. “It was my reputation crumbling away, bit by bit. And the affection for the job eventually followed it.”

What’s so weird about all this is that Rove wasn’t the leaker, Armitage leaked her identity.
BTW Rove disputes McClellan’s account of “secret” meetings with LIbby.
It sounds like McClellan bought the leftwing blogospheres wacky conspiracy theories about the Plame affair even though he knows now that it was Armitage who leaked her identity.