It was originally thought that it was a reference to supercharging. It was defined in a dictionary in 1911 as a "horse injected with a narcotic to make it run faster." So the soup was the narcotic.
Considering people talk about the use of steroids etc as being "juiced" I could easily believe this. Lots of things we say for certain things have very old origins.... F.U.&.* for instance.... lol.
