Since todays patch, if you queue for a dungeon, you will automatically be sent into one after about 90 seconds of waiting, even if there is no one else in queue. Basically it seems that the queue system doesn't see that the player requirement has been met.
I hear this is happening with pvp games as well but not exactly the same (for example I wasn't able to start akram arena by myself), however others have told me that they queue for draconis peaks and sometimes get sent into a game as the only person on the team until others eventually join in.