Sometimes the reason for the delay is obvious to everyone. Years ago i was sat on a 757 doing a short domestic flight in the usa and that particular aircraft had been brought over from maintenance to replace the original aircraft which had gone tech earlier. There was a passenger in an overwing exit seat who was having a seatbelt issue, namely it was stuck in the overwing exit. Along came tech person who removed the exit and managed to deploy the slide. From the flight deck came a rather resigned crew member saying basically that the slide would take 4 hours to repack so please gather your belongings and leave the aircraft.