See my colleague Robert Readfern's guide to Traffic Commissioner inquiries

The Commissioner must hold a public inquiry if consideration is being given to taking regulatory action against an existing licence and the operator requests a hearing. Additionally a public inquiry must be called where the Commissioner is considering taking regulatory action against a transport manager’s good repute and/or professional competence.

https://www.ashfords.co.uk/news-and-events/general/a-guide-to-a-public-inquiry-by-the-traffic-commissioner