or the comment is just stupid and irrelevant.

or it puts forth a argument that's coherent but ultimately stupid theorizing, like conspiracy nonsense does.

both which i guess fall in the 'bullshit' classification.

I too sometimes ask about downvotes on other people’s comments, but it feels a bit like bursting into a room and asking everyone to stop and take a survey so I try to be judicious about it. Usually it’s pretty self-explanatory but every so often I come across a real mystery.