User1's answer gets downvoted, user2 comments on the answer's quality. User1 answers like this:
"You have not put any effort in helping this newbie, and you reward my effort with -1??? If you have simply asked for elaboration I would have done it, but with -1 I will now never do it."
Note that it's nowhere said that user2 was the downvoter. A few minutes after that comment user2's top question gets downvoted.
Given the timing, and the fact that it's a good question, which has no good reason to get downvoted, IMO this is a clear case of revenge downvoting. I flagged this comment from user2:
"thanks for the revenge downvote on [this question]!"
Can't blatant revenge downvotes be reversed? Especially for the given question it looks ugly; it now has 24 upvotes, 1 downvote. And, of course, it's unfair that such negative behavior should be rewarded.