READ AT YOUR OWN RISK (aka there might be spoilers)
SUPERNATURAL SEASON 8 OFFICIAL DESCRIPTION
Season Eight begins with Dean's seemingly impossible escape from Purgatory, though Castiel is nowhere to be found. However, Dean isn't alone. He's returned with a mysterious comrade-in-arms whose reason for escape is perhaps more than Dean bargained for. Meanwhile, Sam finds himself reconciling the life he discovered while Dean was gone with Dean's sudden reappearance. As the boys struggle with their unexpected reunion, they make a shocking discovery that could lead them on a deeply personal mission to settle old scores. If only they could agree – is this a "family business" or isn't it?
Ok so it would appear the Sam was NOT the one who got Dean out, and Dean has no idea how he escaped so could it be that Cas found a way to get Dean out? It's possible since he disappeared on him at the end of last season. Cas might have stayed behind as a form of self punishment, like he thinks he can't do anything wrong to the Winchesters if he stays in Purgatory. Also Dean's new "comrade-in-arms" might have made Cas feel like he's losing Dean's friendship so instead of getting them both out of Purgatory he rescues Dean and his friend instead.
Remember back to episode 7X22 " There will be Blood"? the Alpha Vamp says to Dean "see you next season" now flash forward to season 8 and it hints to Deans new "friend" being a vamp…..
PROMO VIDEO LINNNK
www.youtube.com/watch?v=kEzytq…Now to the bit (~ second 24). Of Cas by the river, it just struck me now , the way he's sitting , like he's remembering walking into water not so long ago and leaving Dean alone. You can kind of see Dean in the back ground maybe?