This has been a common occurrence since roulettes were added.
It's more efficient to leave a dungeon that will take 30-40m with a new or unsynced player, eat the 30m penalty while doing something else and then insta-queue when the penalty expires. A year ago, there was no such penalty. This led to dozens of revolving-door tanks fishing for easy dungeons/trials - especially in low roulette.
Even after the penalty was added, you still have tanks doing things the old-fashioned way, but there's also a new strain of wankers who will fake d/c or stubbornly afk until you votekick them in order to avoid the penalty. You can't dismiss at the start of the duty, so you either wait for enough time to elapse or leave the duty and suffer the penalty yourself.
I've devised what I believe to be a comprehensive solution to this problem:
1) Queue as a tank for low roulette
Please share any feedback you may have.