Date: June 8th, 2025 11:46 PM
Author: JunkoEnoshima
Here's the answer from the screenplay:
MARTY
Doc Brown's all right — he's just a little hung up on time. A couple of years ago, he showed up at my house and hired me to sweep out this garage of his. He pays me 50 bucks a week, gives me free beer... and gives me total access to his record collection — he's got this great old record collection.
(a beat)
Hard to believe he was one of the world's greatest nuclear physicists.
---
Of course, this line didn't make it into the movie, so pedants will say it isn't "canon," but you got a better theory? I'll admit, the screenplay seems to imply that the Doc was planning on sending Marty back to 1955 all along, and that angle, if it's in the movie at all, was severely downplayed.
Screenplay (see page 12): https://www.dailyscript.com/scripts/bttf4th.pdf
(http://www.autoadmit.com/thread.php?thread_id=5734236&forum_id=2#48998379)