I think we should bear in mind, throughout this imbroglio over Scott McClellan’s book, that Karl Rove and Scooter Libby did put him in the unenviable position of telling the White House press corps things that turned out to be false. If he’s cheesed at those guys, you can kind of understand why.
As for why he never lashed out before, I think Tucker Carlson (see Greg’s post below) nailed it when he called McClellan “a beneficiary of that weird Bush affirmative action program for the extremely loyal.” In fact, McClellan was loyal to a fault. If he felt this way during his time as an administration flack, he really should have resigned.
Instead, reading the excerpts of his book available at WSJ.com, McClellan reminds me of those people you see (ok, I don’t see, my wife sees) on Oprah or read about in Psychology Today who are excessive “givers” (a.k.a. “doormats”), until one day they finally snap and go off on a long, expletive-filled tirade at the person who just asked if they would pass the salt.