He's stepping down because of the stress. You can't really argue that Kiffin didn't make Urban's job at least a bit more stressful.
I think it's the stress, which none of us true orangeclad soldiers can deny CLK added to his plate. Combined with losing his favorite "boy toy" (with all due respect) and knowing all the pressure of replacing said "boy toy" (Tebow)......IMO.........
Kiffin went after Urban because he was the main competition in the east. He was the man to beat in order to reach the SEC championship game. I would be willing to bet Kiffin's words are a little more tame now that things have changed (unless they hire a big mouth at Florida).