Most likely it's people trying to get into the same duty as their friends. I doubt they do it to purposely troll.

Duty roulette & PvP are incentives to take advantage of the matchmaking system in this way.

In PvP they probably had a group of 3 friends that would let them know when they successfully matched together, so they could have a fully pre-made team without facing another fully pre-made team.

In Duty Roulette it's just faster to play through the content with friends rather than strangers.

It could also be that the tank was looking for an in-progress run that was near the end, for some reason.