Although less in number, there are some users who has a habit of regularly down-voting the questions which they don't like personally. They do not leave a comment for down-voting. They just comment something.
I know that there is no rule like one need to comment before downvote, although it is a friendly gesture.
But the important aspect is the rationality behind the down-voting. There should be some rationale behind a down-vote and it should not be just a personal bias. Is it true?
If true, then how to handle such users?
You can observe that whenever they come to site and browse the questions, a plenty of down-voting happens.
It is not a big issue for users with high reputations, but it is surely an issue for new users and site development.