I don't think he was a secret liberal all along, and it wasn't just pique over a rejected article that turned him against the left for a time.
A lot of people drifted rightward in the Carter years and then recoiled leftwards later on. Berkeley in particular had that effect on many.
Brock was particularly obnoxious because he was building a career on investigating and let his readers down after he got in good with Team Clinton.
You could argue that at some deep level, David Brock (like the guy he gets confused with, David Brooks) wasn't "really" or "truly" a conservative, but there's no evidence his conservative phase was just a scam.
He’s the one who said the reason he originally flipped was their refusal to publish his article. He said that when he was still “one of us,” so you could take it to mean having his eyes opened to the intolerance of the left. But, I’m not inclined to give him the benefit of the doubt or psychoanalyze him.
Besides, I’m just giving a rough overview of who Brock is.