I think he started believing his own bullshit about Marxism - that it doesn't have a rigorous philosophical foundation behind it. That "Marxism" is just the bad people who want to test down "western civilization". I mean, Zizek can be pretty hard to understand even as a Marxist. I think he figured he could do his tap dance routine and when he realized Zizek is a philosophical heavyweight (at least compared to him) he shit his pants but realized he couldn't back down.