He came out hunting for a KO in the first and was putting everything into every shot. Let's face it, he probably KOs anyone he has ever faced in his life besides Nate with those shots he was blasting him with.
I think he was gassed by the time he got rocked. That isn't an excuse, just an observation. Conor game planned very poorly for this fight. He wasn't using body shots (kicks or punches)..he was loading up on big shots on Nate's chin which is great but Nate was eating them.. and worst of all once he tired he was simply standing in front of Nate flat footed with no head movement and his hands down. Of course he was going to get rocked with all of these things happening.