Once they are in the inventory, and you have several of these (or other containers), double clicking these will open a window, and at the bottom you have an extra button 'Open another' (or some such). When opening these, it can switch out items in your belt slots. If the last opened Chest contains an item that can go into the said belt slot and that item drops into place where the CHEST was sitting in your inventory.
I don't know if that is intentional, a bug, or just poor programming (e.g. linking the button to the chest spot inventory and commencing a double click)... but it is annoying as hell...
Comments
MODS: please toss this message into the trash bin, as it no longer matters. I am done with the game as it is now...