The idea that something was going on with the Klingons sounds good.
So going off of this:
47(!) Klingon ships were destroyed--which sounds like a mass destruction in battle (a la Wolf 359) rather than a hunt-them-down-one-at-a-time type of thing.
Whether the Klingons were going to attack the Federation or were defending themselves from the Narada, they would be gatherng. StarFleet, unaware which this was, would be occupied with that and rendevous-ing at the Laurentian system, leaving only those 7 ships and a bunch of cadets behind.
Digressing a bit:
I still don't understand how cadets were selected for assignments. Chekhov was only 17--too young to be a senior cadet. And there were plenty of cadets witnessing Nero's drilling of San Pancho Bay--so not all were assigned. ["Pancho" is a nickname for "Francisco"]
And...I can kinda understand why Kirk would be jumped, what is it, FIVE ranks to captain. The Kobayashi Maru was to test how a potential captain would perform in the face of fear. Kirk passed that test in real life...repeatedly. I'm wondering if Spock Prime's mindmeld with him gave him a shitload of other info/confidence/etc, so that this is turbo-Kirk; or maybe Spock Prime could control the info that much?