Thanks to a Tip from Twitter User AshWayling, Frostmourne models appear to be covering the floor of Orgrimmar on the Oceanic Realm Arugal. While this phenomenon isn’t happening in other cities or on other realms, the reason for this strange occurrence is still unknown.
Edit: Looking back at an archived forum post, this apparently happened to some realms in 2011 during the Cataclysm expansion.