Wikipedia went down for thousands of users on October 21, and two unrelated services, the Internet Archive and Asana, started misbehaving at nearly the same time.
According to Downdetector, which tallies user-submitted problem reports, the spike began at about 10:51 a.m. ET (3:51 p.m. BST). By the time of writing, US reports had passed 3,000 and were also climbing in the UK. The Internet Archive and Asana outages appeared to start around 10 a.m. ET, slightly earlier. Wikipedia was the worst hit of the three.
Users who were affected saw an error page reading "please try again later". One user wrote that everything except the index page failed to load. Not everyone was affected, though. The site worked normally for at least one person in the UK, even as UK reports rose.
The cause is unknown. Downdetector counts reports rather than confirmed failures, so 3,000 is a signal, not a measurement. It is also notable that there were no current issues at AWS, Google Cloud or Cloudflare. A failure at one of those providers is the usual explanation when unrelated sites fail together, and this time it was ruled out, at least for now.
That leaves two readings, and the evidence does not yet separate them. The three services may share a dependency that has not been identified. Or the timing may be coincidence, a possibility the source itself leaves open by saying it is unclear whether the outages are related. A nonprofit encyclopedia, a nonprofit archive and a commercial work-management tool share little obvious infrastructure, which is why any shared cause, if one exists, would be worth naming.