It could be something as simple as prioritizing the steps of faith trial, considering finishing the full line of msq is required for heavensward and this new trial is part of it. Something like...

-Player A queues for odin trial
-Player B C D queues for trial roulette and gets assigned to A's group
-Player E comes and queues for step of faith
-System would immediately shift B C D to E's group first because it thinks it's more important

Though I kinda think could lead to groans and facepalms from many when they experience something akin to "i queued for low roulette 300 times and it keeps giving me tamtara!"