Obviously, the time continuum has been disrupted, creating this new temporal event sequence resulting in this alternate reality.
8 seconds sound clip from the Back to the Future Part II movie soundboard.
You can hear this line at 00:52:14.557 in the Blu-ray version of the movie.
Quote context
[...]
- The place was boarded up, shut down, so I broke in and borrowed some newspapers.
- I don't get it, Doc. I mean, how can all this be happening? It's like we're in hell or something.
- No, it's Hill Valley. Although, I can't imagine hell being much worse.
- The lab is an awful, awful, awful, awful mess. Attaboy.
- Here, here, here. Let me illustrate.
- Imagine that this line represents time. Here's the present, 1985, the future and the past.
[...]