Sorry, but even if the DPS are being jerks and pulling mobs you should be at least trying to keep aggro on them. The mentality of 'well, you pulled it, its yours' is just stupid and all anyone else see's is a tank not doing their job. If it's something that happens consistently or gets you killed, then tell the party, politely, not to pull. At the end of the day, it doesn't matter where the mob comes from, only that everyone is doing their job properly.

A tank who's not tanking, regardless of where the mob comes from, is not doing their job and it's no surprise they get kicked. Two wrongs don't make it right, so do your job and the blame for the extra mobs can lie solely on the idiot DPS instead of landing on your shoulders.