Traveling with the Deutsche Bahn (German railway company) is always an adventure. Don't expect to be punctual or in time. A slight delay is totally normal. Here are some reasons:
  1. a fire in a wagon
  2. Cows broke out of a pasture and standing now on the tracks
  3. Train was lost
    it took the wrong way and is now figuring out how to return
  4. Bomb disposal on the tracks
  5. Accidents with cars, animals or people
  6. Sudden construction work
  7. Soccer fans/ hooligans destroy the interior
  8. Crazy people getting naked and run through the wagons
  9. Train is stopped and searched by the police
    But nobody knows why
  10. Oberleitungsstörung
    I don't know what this actually means but I believe it's the reason for nearly all of my train delays.
    Suggested by @sarvin