After seeing the gif I'd agree with Quivorir. It's a slip but with a lot of involvement from Rockhold. It's not what you'd generally think of when you hear 'slip' as it's basically an accidental trip by Rockhold. Tripping Machida wasn't his intention as he was getting off to the side to counter with the right hook. He would have been focused on avoiding the punch and countering rather than getting his leg in a position to trip Machida.