tracking iOS performance on GAC is a pain, that's true. ODM is supposed to help, but I haven't heard yet of many success stories, also I'm not sure on which stage they're currently with it since they announced the release last fall. it's basically guesswork and basing on your guts.
The most important factor impacting category rankings is your Downloads Velocity = the amount of downloads your app gets within a specific timeframe, most likely that's last 24hrs or so. So maybe you had a drop in the overall downloads volume?
Nevertheless, unless you're a game, category...