Or maybe his "problem" was that he kept taking fights in the top guys of the division, and no one wins them all. It's not like it's a steep decline, he got annihilated twice by Sea-level Cain (and spinning kicked Hunto between those blowouts), beat the crap out of Stipe, and then got knocked out by Reem. That's not that big of a deal, Reem has knocked a lot of dudes out. Then he beat a juiced up Big Ben, and got knocked out by Stipe.
I'd say his biggest downfall is that he popped. The dude's only four losses in the UFC are to Cain x2, Reem, and HW champ-mode Stipe. That's nothing to be ashamed of.