Downvotes on comments need a little bit of karma. Now downvotes on submissions, those are not normally doable by random users.
unless this is just a glitch of early ranking to get posts onto frontpage that posts start with an invisible negative amount of points and get upvoted out of new and the invisible negatives to frontpage? (eg all posts start out with some value of negative points between -10 and 0 or something and the new votes will help nudge it out to the frontpage?)
If anyone's interested, here's a list of undocumented HN features/rules (although I don't know how up to date it is): https://github.com/minimaxir/hacker-news-undocumented
Now, the post linked itself has been upvoted so it doesn't have negative submissions but it seems to have had been the case.
I don't quite understand how posts can have negative points :-/
A hunch but maybe a post which got confused as comment id of some other comment which got downvoted so you had negative karma, but I am unsure of this spooky action at a distance. It would be really interesting to know the reason behind it.