I swear there is some kind of downvote bot on HN - reasonable posts are often down-voted grey within the first few minutes (like this one's parent), and then eventually climb up to a reasonable place, when, what feels like, the humans have had time to see it - has this been noticed or discussed before? I'd be curious to know if some accounts are serial downvoters, especially as soon as comments go up - does HN look for that kind of stuff?
Of course part of the answer is that new comments have no votes so a quick downvote will make them grey, but there are frequent strange cases.
One thing for sure is that a lot of people downvote based on disagreement rather than a comment's quality - which in my opinion is not right (and I think against the intent of a downvote), but that's a different issue.
Dang said recent-ish* that upvoting inappropriate comments can cost you your voting privileges. Additionally, there is a karma threshold for downvotes. IIRC, you need 500 karma.
Between those two facts, I think downvote bots aren't a terribly likely explanation. You would need to establish an account with downvote privileges and then hope the mods don't revoke its downvote privilege in short order. It seems like a lot of work for probably not much pay off.
That doesn't mean there can't be downvote bots, but it just seems to me a much more likely explanation is people cruising the Comments page.
I'm fairly sure there are multiple downvote bots operating on HN. Frequently, certain comments to topics considered by some as "political" on HN will get an immediate 2 or 3 downvotes in the first 30 seconds, and then over the course of the next 2 or 3 hours, be upvoted back up to black.
Although it's possible that there's some confounding happening here, such as the handful of people who hit refresh on HN hundreds of times a day are also ones who can't stand certain political viewpoints. But it seems unlikely. Far more likely is that there are a handful of downvote bots in operation on a keyword basis.
It's not a huge deal to me, but it does seem kind of obvious.
A downvote on a 4-day-old comment is unlikely to be a bot, since it's likely that no one will even see it. If you're seeing agreeable comments being downvoted after several days, it's probably just a grouchy misanthrope or two catching up on HN on their day off.
Is not possible since they turn downvotes off after 24 hours.
/pedant
As someone who incessantly refreshes HN and spends a fair amount of time on the Comments page, I see no reason why it wouldn't be people seeing new comments to older discussions via the Comments section rather than bots. Although I am a demographic outlier in multiple ways, I cannot possibly be the only person routinely cruising the new comments section.
Good point about the new comments feed. I think your parent either misspoke or misunderstood their parent's "[downvoted] comments on 4 day old conversations", which would mean a new (down votable) comment on a thread that started 4 days earlier.
Similar behavior appears to happen with quality of comments. Initial comments on a story are often insubstantial, knee-jerk low-quality comments. With time, things generally get better.
From what I gather (but I don't have any comments at hand), the mods do pay attention to voting behavior, both to detect up-voting rings and to penalize those abusing downvotes. Their usual recommendation, when coming across a comment you think is unfairly downvoted, is to silently apply a corrective upvote.
Of course part of the answer is that new comments have no votes so a quick downvote will make them grey, but there are frequent strange cases.
One thing for sure is that a lot of people downvote based on disagreement rather than a comment's quality - which in my opinion is not right (and I think against the intent of a downvote), but that's a different issue.