My guess is whatever system downdetector uses to "detect downtime" relies on either GCP or Cloudflare (also having issues at the moment: https://www.cloudflarestatus.com/)
Aren't some of these sites partially based on hits (because of the assumption that if enough people are suddenly googling "Is youtube down", then youtube must be having some sort of issue.
I could see a big outage like this causing people to google "Is AWS down?"