There could be many reasons for downvote, but it is not possible to find out who did it and on which question on purpose as already said. Without explicit comment, the downvote is not very helpful. The topic was already discussed several times, mostly on the Question "There are no dumb questions!". I recommend it to read to everyone who is downvoting or who receives downvote and wonders why.
It looks like there is always somebody who does that. We can probably find the reason for that in the award system, which motivates people to post here, but does not really enforce the quality of the posts (you are awarded for the question and answer itself with significantly more points than for the votes). The point from here affect PGC badge, nice-shiny thing that many people want to improve. Posting here seems like the easiest thing to get points, downvoting can be simple way to "jealously take away some points from someone who does not deserve them".
Also the downvote mechanism does not enforce the quality at this moment. Since there is no penalty for downvoter so he can just throw downvotes all over the place without any penalty. The good solution, as it is used for example on StackOverflow, would be to adjust the pointing system, that it would also remove some points from the downvoter so before doing so, he should really consider if the comment is not more appropriate (since it is for "free").
My proposal would be:
- Update the awards for posting to lower amount for the post itself and higher amount for the votes received (to honour useful posts)
- Update the downvote to remove also some points from the downvoter
Disclaimer: I did not downvote on your question or answer yesterday. Though I did on this one since it is really an answer which does not bring any value, which I hope you will understand.