HunchBluntley: Do you have a trading post? If you had recently made a trade using a bunch of food (generally a bad idea, unless you're trading for different food), and you didn't afterwards reset the amount you want the trading post to keep on hand (which is NOT counted as part of the town's inventory), then the workers from the post might have gone out and grabbed up large amounts of food to transport back to the trading post to try to refill the stock there. That, coupled with various citizens choosing that moment to take food from the markets/storage barns to restock their homes, could probably account for such a sharp drop. But I can't say I've ever noticed such a large decrease myself.

HunchBluntley: Hmm, no idea, then. I have a stupid question: Are you sure it went from 25,000 to 1,500? Because 2,500 to 1,500 could easily happen (and frequently does for me). Also, what speed were you playing on at the time? If it was x10 speed, maybe it was running so fast that it registered everyone taking food over the course of a "month" (e.g., "early winter") with one update at the town hall that showed the food dropping by 23,500 instantaneously. I'm pretty sure it updates in real time, though, so unless the game was lagging badly for you during that play session, that's probably not it, either.

I guess just keep an eye on your town inventory (it's advisable to keep that window open most of the time anyway) and see if it happens again -- and if it does, maybe pause the game and have a look around town to see what the various people and businesses might be up to.

Something just occurred to me before I could post this: How many (staffed, working) taverns did that town have? Those can draw off a fair bit of certain food products.