6 days now, quite the hot streak. Everyday for both of my characters the random daily ends up being ruins of mazzatun. To make it better, it always pairs at least 1 dps that is under level 15. Seriously?
The DLC dungeons can be played by characters of any level. The base game dungeons usually have a minimum level before player can enter. Therefore, DLC dungeons like RoM and ICP have a much larger group of players (basically, everyone in random queue) and a group can be completed for DLC dungeon faster. So you will keep getting thrown into DLC dungeons because they are the dungeons that are most accessible to largest number of players.
This happens on my Tank too, 7/10 times I get a dungeon from IC DLC. Don't have Shadows yet, so I imagine I'll get 10/10 DLC in random when I buy that.
I think the main reason is that a player is trying to farm the dungeon and have selected it specifically, and since not everyone has the DLC, he remains at the top of the queue waiting for someone with the DLC, so when you who have the DLC, you get straight into that team.