Why did a PullRequest have no pipeline triggered?

Prow and Lighthouse use an OWNERS file stored in each git repository to define which developers are allowed to review and approve changes. You can even limit those roles to different folders.

If a non-reviewer submits a Pull Request it won’t trigger CI pipelines by default until a reviewer adds an /ok-to-test comment on the Pull Request.

If you have public git repositories this also avoids the security issue of a non-approver submitting a Pull Request to change the pipeline to email them your security credentials in the CI pipeline.