I've read recently that it was cover for the switch from sucrose to high-fructose corn syrup as a sweetener - when they switched "back" to HFCS, most people never noticed they hadn't actually switched back to the same thing. It's plausible, but as I'm not sure of the timing (I never looked at ingredients back then) I don't know if it's true.
Debunked by Snopes. Coca-Cola already had HFCS five years before New Coke. Here's the link, and also follow the "Cokelore" link at the top for the whole category.
