Could be, some batteries are only warranty for a year now, and factory new ones, they die in 3 years mostly on me. So did you get your car in early 2012 and now the battery is 2-1/2 years old? I put in AGM batteries on my Truck and last non-mustang car when the factory ones died at 3.5 years on the nose. So far have not had either 'stang that long yet.
Edit: DagNabit, the Sears Gold AGM isn't made for a 2013 mustang
. Now I'm on a hunt for an AGM for when that day finally comes. They do have a good 590 Cca lead acid mode tho FOR $99... but I like the AGMS for trucks siting in cold below zero airports for a week while I am in FLA