I noticed that too, when he got clocked in the Carwin fight I believe it was, either that or the Cain fight, his head just bounced back and it looked like he was done even though the punch wasn't that right on, it definitely looked like he wasn't expecting that.
I know where your coming from but I don't think Lesnar was a 100% in the Carwin fight, it was his first fight after a year, not only that but the guy was on his deathbed months before that fight, I know as a UFC fighter that isn't an excuse to make but still you gotta realize it.
Cain is pure garbage IMHO, I have no idea how Lesnar lost to him, In my book Carwin was the only legit threat to Lesnar, I don't think Lesnar even took Cain seriously (heheheh I still remember that racist Taco Bell comment he made, I get that it was part of the build but I'm just saying Lesnar was as confident as he ever could be after beating Carwin, going into the Velasquez fight, he's probably thinking "this guy can come at me all he wants just like Carwin did but who am I kidding, I'm gonna beat him in the end").
Cain is the furthest thing from garbage. He has laser point accuracy with his punches, his wrestling is excellent and he out works the 170 pounders in the gym, his cardio is insane. And I fully believe Lesnar took him lightly, it was obvious. Lesnar is a lot bigger than him and you could tell he thought he would just out muscle him, hold him down and beat him that way. When Cain popped back up, Lesnar didn't know what to do and got blasted. Which brings me back to what I said before, he needs to work on his striking in case taking guys down doesn't work. Lesnar is a fucking silver back gorilla and there aren't a lot of guys that can deal with his combination of speed and size. But Cain did, he beat the fuck out of him. If Lesnar reacted to getting hit differently, he would give tons of guys fits.
And Lesnar not being 100% going into the Carwin fight had nothing to do with it, he still reacted the same way when Cain hit him and I guarantee you Cain doesn't hit as hard as Carwin. And Carwin blasted him from the get go in their fight and as soon as he got hit, you could see the look on Lesnar's face like "Oh fuck, I didn't think he could hit that hard". But he took everything Carwin had, something no one else has done. So his chin is fine, it's just his reaction to getting hit. He freezes up, runs and then they just swarm him and the ref has to stop it.
Brock will never beat Cain, in my opinion. I think JDS would give Brock fits as well.