If the number of negatively tagged users keep increasing, it will reach to a stage that majority of the users in the forum would be negatively tagged and hence could fault the essence of the trust system.
I think you're just seeing clusters of shitposters for 1xbit or some other shady campaign or members of an alt ring that got busted; I don't think the level of tagging has increased drastically over the last few months or even years (though I could be wrong, as I have no data on that).
Keep in mind, though, that there are thousands of active accounts on bitcointalk and the chances that the majority of them are going to have some kind of DT tag is fairly slim IMO. Also keep in mind that there are a
lot of scammers and otherwise dishonest people who frequent the forum. It's been that way for a long time now.