It's been a little while now, but on April 26 Google apparently updated the rules that determine its search rankings (known as the algorithm).
That said, this time isn't anything special—Google frequently updates its algorithm in order to provide searchers with "better information."
Recently, since around the end of last year, following the Welq issue at DeNA,updates targeting low-quality curation siteshave been the main focus, and this one appears to be part of that as well.

To put it roughly, a curation site is an aggregator site. When it comes to aggregator sites, NAVER Matome and the like are well known.
It's not that aggregator sites themselves are bad, but problems arose with copyright—copying (that is, ripping off) other people's blog posts and photos without permission—and with huge numbers of articles that were untrue (that is, fabricated), created simply by stitching together multiple articles.
In particular, Welq, an information site dealing with medical information (currently suspended? shut down?), became a major issue because it was full of untrue articles… fake news.

In response, Google has been working on rules to eliminate such "clearly misleading, low-quality, or offensive content."

Reference: Recent quality improvements in Google Search >

With this update, several of this website's search rankings rose significantly. The biggest changes were:
※ Hamamatsu website production … 15th → 5th
※ flyer production Hamamatsu … 10th → 1st

From April 25 to 26, our rankings rose for the search phrases above and others. Since we hadn't made any major changes before or after, I think the update may have had a positive effect.
That said, in our case we know these search phrases don't lead to results, so honestly it's a bit iffy (^^;)

Well, these days going all-out on search optimization often backfires. Steadily providing good content—nothing beats that.