In my case, it was the ingress running out of subdomains because each staging environment would get its own subdomain, and our system had a bug that caused them

In my case, it was the ingress running out of subdomains because each staging environment would get its own subdomain, and our system had a bug that caused them to not be cleaned up. So the CI/CD was leaking subdomains, eventually the list became too long and it bumped the production domain off the list.