Why? Because of how good he is taking down wrestlers (re: couldn't take down 240lb couture)? Or his proven knock out power (re: has terrible standup)?
I don't think it'll be an easy fight but I see Carwin hurting Lesnar and then capitalizing it with a TKO, 1st or 2nd round.
Brock took Couture down when he tried with ease, in the Couture fight Brock was working on stand-up more then wrestling, because Brock knew he could take Couture down if he wanted to. Brock's KO power, we have not seen if he has any yet. But he did drop Couture when he barely hit him behind the ear, and he dropped Mir with a light punch nothing to hard. Yes he rocked Herring but it was around the eye and not near the chin so he didn't KO Herring but did break the eye bone. Every fighter Brock has fought look bad after the fight except for the very first MMA fight Brock had. I think everyone will see Carwin bleed and get controlled. Carwin will run out of gas in SECOND round you watch.