Misha told EW that is, in fact, when Dean is exiting Purgatory. And there's much pain and confusion associated with it:
“That very moment is a pivotal moment. I think Dean is confused about what happened at that moment and won’t actually find out what happened then until episode 7. It’s going to take a while to see that moment play out.”
Once Cas gets topside again and is reunited with the boys, however, he sees his return as a chance to right past wrongs:
"[Cas is] reinvigorated with a sense of purpose. He essentially feels like he has to do good in order to right the wrongs of his past. So he starts to collaborate with Sam and Dean for that reason — so that he can continue to try to make things right. He’s no longer paying penance, really, because it’s no longer an act of suffering for him. But he’s doing his best to right the wrongs of his past.”
But this being Supernatural, things don't go very smoothly:
“He’s doing that in ways that help Sam and Dean and then, ultimately, it might also make life — as per usual with the Supernatural universe — complicated for Sam and Dean. At first, it seems like everything is hunky dory. But things quickly fall apart.”
What do you think happened between Dean and Cas when Dean left Purgatory?