It happens after battlegrounds, WG, even regular dungeons. I came out of my daily dungeon, went afk in stormwind for like 30 minutes, and when i came back i was in pvp mode, and could not log out instantly because was not rested. But it seems only happens in mayor cities, because if i ride over to Lion's pride inn in Elwyn forest I get rested. If i ride back to stormwind I'm not rested.
Has also happened in Iron Forge.
