I haven't seen the pattern you've described, but I have noticed that many downvotes seem to come to in twos. It always strikes me as a bit suspicious when that happens suddenly, but I try to avoid paying too much attention to the score because I consider it a toxic antipattern.
> Then all of a sudden, some of my most recent comments (in different threads) get downvoted at the same time

What this typically means is you made some comment that (whether because it was seen as good or bad, and maybe both by different users) got people to click through to your profile and past comments, giving added attention to comments that otherwise would have flown under the radar.

If people were stalking you, rather than a sudden reaction to a particular post, you'd expect consistent downvotes as you posted things, rather than focussed flurries on groups of older posts.

I had that happen to me a long while back. No idea if that has happened recently as I use uBlock filters to remove my view of karma. I found these filters here on HN submitted by another member. This assumes you have the uBlock addon installed.

In Addons -> uBlock Preferences -> My Filters

    ## HN Block Karma View
    news.ycombinator.com##.comhead .score:style(overflow: hidden; display: inline-block; line-height: 0.1em; width: 0; margin-left: -1.9em;)
    news.ycombinator.com##.comhead:not(.sitebit):style(overflow:hidden; display: inline-block);
    news.ycombinator.com###hnmain > tbody > tr:first-of-type table td:last-of-type .pagetop:style(font-size: 0!important; color: transparent!important;)
    news.ycombinator.com###hnmain > tbody > tr:first-of-type table td:last-of-type .pagetop > \*:style(font-size: 10pt; line-height: 1.45em;)
    news.ycombinator.com###logout::before:style(content: "|"; padding: 0.25em;)
    news.ycombinator.com##form.profileform tbody tr:nth-child(3)
I have no idea, but kudos to this being one of the problems in your life, having conquered all others. :)
Well, pretty all of my attempts at humour get downvoted and anything which is controversial in the wrong way gets zapped as well. I’ve tried to answer some topics “honestly without snark” and watched it go negative. This leaves me thinking I’m not quite in sync with the readership as a whole.

But the up/down voting system can be frustrating as it can be done without comment. Often, you have no real idea why you did well or poorly.

As a guess, the readership tends to like tech stars, the business and neat programming stuff. It dislikes government, left wing politics and criticism of tech stars.

Sometimes, if I made someone mad enough. I think some people just get so frustrated that they can only downvote a specific comment once that they vent their spleen on your comment history.

Given how vituperative and hostile this place can be I wouldn't be surprised if someone actually created a network of alts and built up their karma and created a mass downvoting botnet. I wonder what language they used.

You're not the only one; It happens if you tell jokes, go against a herd narrative (with counter evidence) or just disagree. But really at this point, who cares.

It really doesn't matter. Downvoting doesn't mean anything unless a direct reason or reply is given. Most of the times the ones that do it to me are sometimes having a bad day or just very angry.

So it's fine, it happens, just ignore it.

You probably made some political (or otherwise sensitive) comment, which triggered someone.
I downvote nearly every comment I see by default. It makes the few I upvote that much more special.
Hackernews has a downvote function? Genuine question.
Attention to yourself is always good, no matter what form, positive or otherwise.