I just took it for granted, until I tried to explain it to a kid.
I think it’s short for “bouncing off the walls” as in a padded cell in a mental hospital.
From American Slang by Robert L. Chapman
The Phrase “OFF THE WALL”
Where did it come from? In 1972 I was living in Chicago, walking down Garfield Boulevard with a close friend from Gary Indiana. I asked him a trick question. Then, I asked him; Do you know where I am coming from? He answered yes, but he really didn’t. That’s when I answered him “Off The Wall.” Meaning "crazy, foolish, tricky, joking, or nonsensical. The phase exploded and caught on like fire. He took the phrase back to Gary Indiana with him. That’s probably where Michael Jackson got it from. Now that’s the honest truth. We still get a blast out of seeing how the phrase has evolved over the years.
AMERICA CAVER
Wow, after 12 years we finally have a definitive answer. Certainly worth the wait.
The phrase first appears in print in the familar sense in Trumbull Park, by Frank L. Brown. (1959)
That is mildy suggestive of a Chicago origin, but it seems that our respondent is consistently twelve years late to the party. Maybe something to do with the Lunar calendar? 
NO! When I was in kindergarten in 1965 we had to spend recess sitting on the wall surrounding the playground. Any kid who got down to play on the equipment would be sent to the principal’s office and given detention for being “Off the Wall”. The phrase evolved to refer to someone not behaving like everyone else.
You know it’s true, because I’m ending with “that’s the honest truth.”
ETA: Dang, beaten by someone while I was making up my origin story.
Come on, why be skeptical. If America Caver said he just said it one day for no particular reason and those words somehow supposedly had some inherent unexplained meaning and it caught on and became used around the word, including decades before he came up with it, it surely must be true.
Hey, a trick question has to have a trick answer.
After all, his post is his cite.
Jackson’s album wasn’t the first of that name. There was an earlier one by Smokestack Lightnin, but that came out three years before you thought of the phrase. This could have had something to do with too much acid.