What an update does, and does not, tell you
Google changes how it ranks pages all the time and announces the larger changes, core updates among them, with the dates they roll out. A fall that begins inside that window, across many pages, is most likely the update.
It does not tell you what was wrong with the pages. Google's own advice after a core update is to look at the pages that lost and ask whether they are the most helpful answer to the searches they lost.