> upvote downvoted comments that I think are perfectly fine
I can't speak for what you are specifically upvoting that gets so many downvotes, BUT it's entirely plausible that what you think is fine is against the guidelines, and hence an interpretation for "being controversial".
Good example (that you may or may have not done, again, I don't know):
Throwaway accounts are ok for sensitive information, but please don't create accounts routinely. HN is a community—users should have an identity that others can relate to.
Let's say you see several anonymous "green" posts that get downvoted into oblivion but find those posts beneficial to the discussion (which is very plausible btw). Well, as per the guidelines, just creating these anonymous users is a big no-no. Hence, I would deem someone who continually upvoted those as trying to spark controversy - even though their upvotes were probably very innocent.
I've been on HN for a very long time - it's hard to follow the guidelines btw.