Sorry call me stupid, but why are aircraft heading to the engine bay to hold once they filled up with passengers and left the stand?
Surely the aircraft is then heading to the runway for the departure?
With people having to be at the boarding gate only 30 minutes before departure and airlines like easyJet and Wizz having a 30-40 minute turnaround... It is not like they are getting the passengers on the aircraft extra early, or the aircraft is hogging a stand for 2-3 hours like you might find at Heathrow when an aircraft is being turned around for a long haul flight and has 400-600 passengers to board.