Custom cause action returned from 
SCMSource#retrieveActions(jenkins.scm.api.SCMHead, jenkins.scm.api.SCMHeadEvent, hudson.model.TaskListener)
is not displayed on build page.
Tried with freestyle-multibranch and workflow-multibranch, both contains only "branch indexing" cause, while usually build may have multiple causes.
jenkins.branch.MultiBranchProject#scheduleBuild tries differentiate actions. So probably it should extract and unify revision actions into single CauseAction() if it passed in actions.
Anyway CauseAction is a causeBag and it just additional UI helpers.