He was extremely cocky before his losses in the UFC. You could tell in his interviews/fights that he thought he was invincible and no one could stop him.
Hell, look at his fights against Brock and Bigfoot. He tells the media he will tear Brock apart piece by piece, send him into retirement and make him go back to fake wrestling, etc, etc. Fight starts, he has his hands down the entire time, just walking forward throwing knees. If Brock wasn't so scared of Overeem in that fight, he would've sent Overeem a very big reality check. Brock didn't even throw much punches when Overeem's hands were down, walking forward throwing knees lol. He didn't even go for a takedown (He reached for leg once, then gave up on it without even attempting to finish it)
Then you have the fight against Bigfoot. He says Bigfoot is just some random dude they picked as a warm up fight, Bigfoot is not professional and has to have a translator, he'll destroy Bigfoot, etc etc. Fight starts, he has his hands down to prove that Bigfoot can't touch him. And he just coasts through it as if it was a sparring match lol
He got humbled after the Bigfoot fight though. But honestly, I don't blame him for getting cocky. He built up an invincible aura at the time. If Cain was champion at the time, I think more would pick Ubereem to beat him at the time