https://github.com/jenkinsci/blueocean-plugin/blob/master/blueocean-pipeline-api-impl/src/main/java/io/jenkins/blueocean/rest/impl/pipeline/scm/ScmContainer.java is a class annotated with @Extension and calls Stapler#getCurrentRequest in the parameter-less constructor that will be called by extension lookup.

It is an incorrect assumption that extension lookup (and therefore instantiation of annotated types) only happens during HTTP requests.