I wouldn't even call it passive-aggressive; it's soapboxing, plain and simple.
> This article is about X. But I don't care what the article says about X. I have something to say about X, and by God, I'm going to say it.
--But I don't know that this is a particularly problematic thing to be happening in a threaded comment system, since people can get sidetracked by "soapbox issues" in a subthread while letting the "parent conversation" continue around them. It's just kind of confusing for people who treat this place like a linear-chronological message-board that discusses one topic at a time.