I have noticed that when you solo queue and someone 'declines' then it says something along the lines of "somebody has declined, you have been placed at the front of the queue". When you group queue, however, if somebody declines then it says "Someone in your group has declined the invite, you have exited the queue..
It was already confirmed by somebody as ZoS that the someone in your group declined notice could have been any of the 12 participants, but maybe it sees them declining as somebody in your group and therefore puts you to the back of the line.
This isn't the only thing making it break since solo queue can take the *** in terms of queue times and unbalanced matchmaking (e.g. 4 v 4 v 2, 4 v 2 v 2), but it is something I have noticed.
Everyone has a plan, until their plan gets nerfed
Apparently the only Stamplar left on PS4 EU