Wow, I'm even geekier than Bob Gale himself about Back To The Future. It turns out that I printed the scripts of the movies several years ago and there it's written the way they meet each other (there are many changes between the script and the final movie, but those shouldn't affect the way Doc and Marty come across), and it's not the same explanation that Bob gives in the provided link (and I quote de script):
MARTY Doc Brown's all right -- he's just a little hung up on time. A couple of years ago, he showed up at mu house and hired me to swwep 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.
MARTY
Doc Brown's all right -- he's just a little hung up on time. A couple of years ago, he showed up at mu house and hired me to swwep 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.
http://www.imsdb.com/scripts/Back-to-the-Future.pdf
So they meet like that :)
Greetings from Spain, Jorge.
P.S: obviously you can check out what I said in the link.