My personal opinion? Cas. Cas didn't want to get Dean into the Alpha hunt, so Crowley told him he had someone else he could take off the bench, someone even more knowledgeable. Originally Samuel told Sam that whatever pulled Sam up pulled him down, and essentially I think that was true, but Sam was 'up' first. Samuel telling Sam he had no idea who brought them back may not have actually been a lie - he only dealt with Crowley, and when he met Cas they didn't seem to know each other.

*dodges plot holes like a baws*