The notification seems to be automatically triggered on the very first boarding pass scanned. Usually this is PRE pax and I’ve seen very often gate agents gather all the PRE pax and do doc checks and scan the boarding pass and have them wait in a separate area for the go ahead from the aircraft so they can just go as soon as they get the go ahead. This will trigger notifications like this which can sometimes be 30 minutes or more before Z1 is actually ready to roll.