Two main reasons.

One, Im totally convinced he was damaged goods before the Benn fight and I think people who should have been looking after his best interests new it too. The blinking, the mouthpiece hanging out, being chinned in sparring by little guys, complaining of headaches, blacking out. It's all there. He was a ticking bomb. McClellan fought like he had no option but to chin him in the first, like he knew it too.

Two, Benn was super human. He wasn't going to be denied.

A couple of things to mention. Benn didnt get helped back into the ring. There's a difference between standing your ground and putting your hands out so a fighter doesn't roll onto TV tables or the floor, and helping someone into the ring who cant manage it under their own steam. He also didn't get a long count. He was up and ready to fight before 10. Even if you think it was slow, he should have been given 20. He did get extra seconds when McClellan tried to go around the back of the referee to get back at him. That set the tone for the rest of the fight, the referee was fussy, with both of them but especially McClellan.