User blog comment:Commander VB/Mass Effect 3 story understanding/@comment-5439505-20120824230043/@comment-5379655-20120825065029

On a very similar note, the catalyst could not enact one of the three endings without the help of Shepard, showing that he had some constraints in his programming. It's very likely that the process to open the citadel relay was out of the catalyst's powers and required a direct, manual action. It is easy to conclude therefore that the keepers were the Catalyst's means of interfacing with the citadel controls and when they evolved past the operating signal, sovereign was called in with the Geth with the intention of replacing the keepers with the Geth in order to rectify the problem. It's not as airtight as it could be, but I think that's a pretty solid explanation.