Nobody knows. HN has downvotes for a secret group of users. HN weights votes using a secret algorithm, so some users may have the ability to grey out posts with a single click.
Now your post is grayed out.
Someone is watching...
I was tempted to downvote both of you just to cause untargeted suspicion :P
All you need is a certain amount of karma - which you currently don't have. There is no other criteria.