Flightly or flightradar24 listens to air traffic control and updates the app before any announcements are made. Huge help for delays or gate changes.
But maybe the passengers or crew on that plane had a better idea of why they were delayed, which I'm not sure any site would show. I've been on the other end too, knowing my plane isn't taking off due to a jammed door and texting updates to someone who was supposed to pick me up on the other end.