Wikipedia is switching to the HTTPS protocol: all Wikimedia websites – Wikipedia, WikiSource, Wiktionary, and the rest of the organization's websites are switching to delivering their content via HTTPS.
The HTTPS protocol allows a secure connection between websites and a user's computer to ensure data security and integrity of transferred information. Using the protocol also makes tracking users and monitoring their activity more difficult.
This change have caused quite a stir among SEOs – result measuring tools reported "massive changes" while the biggest change was the fact that Wikimedia websites have changed their address prefixes to HTTPS.
This case is not a Google algorithm update, but since Wikipedia is very dominant in Google’s search results, many SEOs have noticed shake-ups and fluctuations that day.