Driver hits and damages roll-up door while backing up to a dock — Preventable or not?

This video is an adaptation of a scenario from CCJ’s Preventable or Not manuals. It is presented via a partnership between CCJ and Instructional Technologies. Truck driver John Doe was backing to a restaurant’s delivery dock when one of his spotters became distracted. Though his other spotter was waving for him to stop, Doe’s trailer crashed into the dock’s roll-up door, damaging it. Was this crash preventable or not?