I do not know if it is rigged, but it should be rigged in that you should be placed in groups for whatever dungeon has had people waiting the longest in queue for. If that means the longest queues are for satasha then that is what DR people should be placed in, if it is snowcloak that has had a group of people waiting in queue for longest time then that is what DR people should be added to. Remember DR is not primarily to service old players who have run it million times before and just in it for the soldiery/poetics. It's primary purpose is the people who need help completing places and/or need help getting people to run it with them due to long queues by putting DR people into those groups to help them while getting a nice bonus incentive for helping.