It isn't random at all. Blizzard is trying to force you to clear all their oh so awesome instances. Once you cleared one, you literally never get it again as long as you got some instances you never cleared before.

Back when you had to discover instances to be able to queue for them, you could cheat the system by not discovering any instances at all, so you had a 50:50 chance for either DM or SFK. You've been able to add any instance you liked by discovering its entrance.
They changed that later on and since you no longer have to discover the entrances, am not bothering at all. Most cata instances are a royal pita and not fun at all imo.