I noticed that this question: Given present-day Earth as we know it, how could I fake a radio transmission as coming from a distant star? Had a downvote on every answer that seemed to be applied in a short time period.
It might (or might not) be less-than-honest voting, but that wouldn't qualify as vote fraud even if it all came from the same voter. When Stack Exchange talks about vote fraud they mean targeting specific users -- getting your friends to vote for all your posts (or using sockpuppets to do so), targeting people you don't like with downvotes, and similar actions. If we see more of a pattern we can ask SE to investigate, but this could be explained by one user who's hard to please, or the question having been seen by a bunch of people in a short time each of whom cast some of those downvotes.
It is a little peculiar, as usually when several answers are downvoted there's a least one that's not, and sometimes that's because the author of that answer is downvoting his competition. That's mean, but it happens. It's called tactical voting. When it's come up in the past the answer has been to just shrug off single cases, but if you notice a broader pattern please let somebody know.
Anybody who's participated for any length of time on Stack Exchange has gotten unwarranted downvotes. It's annoying, but there's nothing to be done about it. Don't invest too much meaning in a single downvote. If a downvote causes you to notice something that can be improved then making the improvement could reverse the vote, but if you look again at the post and don't see the problem, then I suggest shrugging it off.