I would guess that any or all of the possible reasons that you offer could be the case. The main point is that - you will never find out what was the case!
This kind of delay can occur on any day at any airport in the world. It might happen every day and it might happen once a year. It is just the way things are. That sounds defeatist but it is realist! I have been paxing for 40 years and this is entirely normal. Frustrating - Yes!!! But normal when there are so many flights with so many changes in each day's operations.
Flights I have been on have often landed at a time that would give an on-schedule arrival at the gate but ... we then hold off as the gate is not yet free due to a delayed push of the previous. After ten minutes, it is decided to go to a remote stand where we then shut down and wait for a bus. By the time you have collected your bags, it might be an hour from touch down on what was a 90 minute European sector.
Sorry to be a wet blanket but carriers and airports no longer have the spare capacity to ensure that staff are always on hand for any eventuality. It's just about money. Make sure that you have reading material, water, food and any medication that you need. If the a/c is shut down at the gate, they will probably allow use of cell phones to call ahead but check first. If the main systems are shut down and they are running on APU, they will probably allow it even if the doors are still closed.