I don't know what would cause that. As far as I know of there is only one way they can go down. I have not heard of anyone else mentioning anything but not everyone keeps track of the item.
They haven't. The Topic is right above the one you just made. All I did was move it to Offline which to what I understand may be in the Archives tomorrow.
Cached data is information stored from a Website or App that makes reopening that Website or App faster. Cached data reduces load times, but that data has to go somewhere, so it takes up some space on your device. Once you clear (Free Up) that space the Website or Apps may take a little longer to load since the previous information was removed.