One reason electricians may install outlets this way is safety. If a plug becomes partly loose, having the ground pin on top may reduce the chance that a falling metal object touches the live prongs.
In some buildings, an upside-down outlet can also act as a visual signal. It may identify a switched outlet or another circuit with a special purpose.
However, this is not a universal rule. Outlet orientation can depend on local practices and the electrician, so an upside-down receptacle does not always have a special meaning.
