When you queue for "random" dungeon it's not actually random - you're simply placed in the group that decided to queue for specific dungeon and you're the matching filler.
To verify that I specifically started asking "random" groups that led me to DLC dungeons whether they checked "random dungeon" or did premade group of 2-3 people intentionally queuing for DLC dungeon. I've got ~75-80% positive answer about intended DLC dungeon run.