Found it somewhat rude.

I'm just a weekend camper that stays close to home. I've found a small town park near me that has a dump station and no restrictions on who dumps there. I often carry my waste to it. That way I'm not in anyone's way and at the same time I don't have to wait for anyone else. If I can't do that I try to time my leaving to when I don't see anyone else getting ready to go.

I agree with the OP that ; as least as he has stated it; that this behavior was rude. They should have been better prepared to get dumped quickly and then get out of there. Even if they were working as a team.