There are no obligations that someone that downvoted also provide a comment of why they did so. Comments are preferred, but not obligatory, for obvious reasons: If you were pestered into commenting when you didn't like to, you either would've commented something useless or not constructive or you would've downvoted less. Downvotes play a major role in moderation of the community, and discouraging them is blatantly wrong.
Unjustified voting will be dealt with. Additionally, making downvotes obligatory has been discussed to death in meta.SE: Encouraging people to explain downvotes (This one got implemented, while there are numerous other ones that were not such a good idea: 1, 2, 3, 4 and so on)
So downvoters are not by any means mandated to explain the reason for their downvote. However, they're being kind if they comment and explain the potential reason for the downvote.
- Downvotes on meta posts primarily indicate disagreement and are not necessarily given based on the quality of the post.
- Many people get drive-by downvotes every now and then. I don't care about them. However, if the downvotes started piling up, then you should double-check what you've written in your answer.