Maybe there is an API to downvote posts and some bots found it?

Edit: Actually I would wager that it’s some kind of automatic downvoting when the system detects bad votes or comments. Something like “friends shouldn’t vote”. Since we see voting on posts and not comments we could see it go negative.

I suspect there’s downweights the system can apply - and if five people voted it up, the -5 downweight was applied, and then four of the upvotes were removed, you’d end up at -4.