Even if you could objectively review that, HN doesn't have a rule against downvoting for disagreement, pg decided it was acceptable very early on and it hasn't been changed. (although plenty people think it should)
This down-then-up pattern happens on all sorts of topics here. Almost everything I post gets a few downvotes within two minutes or so, then inevitably crawls back upward as people actually read what's said. One change suggestion I'd have for HN is to disallow downvotes within some time of posting, like 30 minutes, give people a chance to read it, and decide whether it is truly not contributing to the conversation.
I assumed the latter, because those places tend to be places worth sticking around, and because this is definitely where HN falls in.
I don't think this is a regular occurence on HN. But on reddit? All day every day.
Personally, I often upvote posts I disagree with, because it is a good discussion starter or just generally a good comment/discussion/disagreement.
If you write a comment that is against what the majority of the site beliefs you will be downvoted. If you agree with the majority you probably won't have experienced this.
I know that if I make a comment that is positive of bitcoin it will get to -3 or -4 in a very short time and then in a few days it will get to +2 or +3.
I don't know that it is a specific group of people who do that, but it is consistent enough that I can pretty much count on it happening every time, and clearly since it earned more upvotes than downvotes the community did consider the comment somewhat valuable.
Granted you get hammered much harder on r/politics or r/news, but the problem is the same.
The solution is also pretty obvious: randomly review downvoted comments and if they are downvoted for disagreement and not merrit, ban the people who downvoted it from future voting. For new users shadow the votes for a while to be sure they understand the community[0].
[0]: No democracy permits people to move there and vote right away. For native born citizens they have to wait 18 years to get the vote.