Someone who wanted to test if they all are still fresh?
Other than that, I think there are enough crazy people out there - gestures wildly at everything going on right now - who’d probably do that, let alone zombified/rioting ones 
…
But yes, it’s not likely.
Well, this can actually be explained logically… Assuming that they didn’t quickly put in fresh stuff just as the Cataclysm happened, this stuff might actually be “old”.
That explanation aside, the shelf life is also temperature dependent. Given your edit of a fire in the building, the heat from it might actually have sped up rot.
If it was caused by a gasoline Zombie (or magic, if you have that mod active) or similar, the fire might has consumed all the fuel in one go and now keeps burning as long as the internal fuel-counter lasts (remember when I said that one drop of diesel/gasoline keeps a fire going for about a day?).