I'm sure the airline had a reason, though at a minimum it wasn't shared with the passengers. In all likelihood that means it wasn't a mechanical issue as those

I'm sure the airline had a reason, though at a minimum it wasn't shared with the passengers. In all likelihood that means it wasn't a mechanical issue as those are always shared in my experience. My best guess was that, given it was the last flight of the day, the airline decided they needed to move planes around differently for the next morning.