This query returns the list of posts that got downvotes on that day. Most of them are now deleted.
The number of downvotes on each (typically 6) together with the fact of deletion indicates these were automatic downvotes cast by Community user when someone flagged the post as spam.
What happened is that a then-new user posted several answers in a row, all of them inviting readers to "check out [their website]". This activity was considered as spam by several users, hence the flags. Some of the posts remained on the site, after the link was edited out by a moderator.
It seems that moderators have a fairly limited set of tools for monitoring voting patterns. Systematic patters of voting involving a single pair of users will be flagged as suspicious, but that does not appear to be the case here. Indeed, the answer in the above-linked quesiton strings together a few pieces of evidence that seem to suggest that, although unusual, the voting pattern is not suspicious.
I read the thread Ubiquitous linked and followed the evidence. There was a Suffrage badge in Oct 23 but it had clearly to do with upvotes (there is a spike of upvotes in that day)-and not "serial upvoting" in the sense of upvoting a single user. I guess the user in question had time in his hands and roamed the site upvoting things he liked. So the downvoting spike in Oct 28 appears to be a collective result- perhaps a combination of a disliked answer posted that day and downvoted by many users, together with the coincidence of a handful of users that decided that day to exercise their voting duties, especially as regards, say, badly formed homework questions and the like... My question was not only about the possibility of non-accepted voting behavior, but also out of sheer curiosity for such an impressive spike... but outliers always exist and they are not necessarily carriers of useful insights -sometimes they are just that: outliers, a manifestation of randomness.