Rosenstein was likely one of the first. With criminal conspiracies, the first one to run to the prosecutor gets the best deal, by far; sometimes even immunity from prosecution. May not seem right, but it helps get all the others to reach a deal, or face a trial in which they will be found guilty. The risk to that person is they have to tell everything they know or their deal goes away. The others charged will be put in a position where they must choose to reach a plea deal or go to trial. Making the government take you to trial most often results in a significantly longer sentence. As more defendants cut deals, hopes of a not guilty verdict will be very low to non-existent.
Astute comments and I agree that Rosenstein probably flipped. He was an insider and probably saw the handwriting on the wall that this whole plot was going to be unraveled and exposed. So he was proactive in offering up info to minimize his vulnerability to prosecution.