The lightning bolt that hit the DeLorean caused a gigawatt overload which scrambled the time circuits, activated the Flux Capacitor, and sent me back to 1885.
9 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:07:29.458 in the DVD version of the movie.
Quote context
[...]
- If the me of the future is now in the past, how could you possibly know about it?
- First, let me assure you that I'm alive and well.
- I've been living happily these past eight months in the year 1885.
- The overload shorted out the time circuits, and destroyed the flying circuits.
- Unfortunately, the car will never fly again. It actually flew?
- Yeah, we had a hover conversion done in the early 21st century.
- However, I've gotten quite adept at shoeing horses and fixing wagons.
[...]