I imagine HN wanted to protect it's community from something they viewed as tangential to their focus but possibly highly vitriolic in content (to the degree that they influenced the submission, which I'm not sure the level of). Alternatively, it could have been a community self-policing, by flagging articles that they thought were poisonous, or that were viewed as promoting misinformation (do the degree that happened. I haven't followed this closely, but that seems to be a common occurrence shortly after a disaster).