There are two general reasons why a hn comment might get downvoted.
First, if you are not contributing something positive to the conversation, most people will downvote you. This is why the comment where you are asking why you got downvoted is now downvoted.
Next, it is generally considered acceptable to upvote comments you agree with and downvote comments than you disagree with. I think this is why the comment you are asking about was downvoted initially.
Note, there is some controversy about downvoting because you disagree. Personally, I feel like this makes hacker news a more uniform and less interesting place than it would be otherwise, but I believe I am outvoted in this matter. On the other hand, upvoting or downvoting is clearly better than content-free 'me too' posts.
First, if you are not contributing something positive to the conversation, most people will downvote you. This is why the comment where you are asking why you got downvoted is now downvoted.
Next, it is generally considered acceptable to upvote comments you agree with and downvote comments than you disagree with. I think this is why the comment you are asking about was downvoted initially.
Note, there is some controversy about downvoting because you disagree. Personally, I feel like this makes hacker news a more uniform and less interesting place than it would be otherwise, but I believe I am outvoted in this matter. On the other hand, upvoting or downvoting is clearly better than content-free 'me too' posts.