I find it hysterical when people say Conor 'gassed' in that fight. It was midway through the second round, with Conor not only hurling bombs, but landing, when Nate groomed him for a clean 1-2. Everything changed after that.
If you rewatch the PPV, they roll slow motion angles of all the shots Nate landed right before Conor dove for the take down. He was getting absolutely battered.
So he had a decision to make - get knocked out standing up, or dive for a take down and try to recover in the process.
Since Nate is a far more competent grappler, he immediately use the guillotine to reverse him and passed his guard to mount. Now Conor has another choice - get TKO'd on the ground - Nate was punching his head in with ground-and-pound, or give up his neck for the submission.
He chose to give up the submission instead of tapping to strikes or waiting for the referee to call a TKO.
All pretty straight forward, all bad options for Conor. He was beat no matter which path he chose.