That was the first year of the heavy metal/hard rock category, which is why it caused such a ruckus. Prior to that year albums from those genres were just never nominated for anything.
I'm sure the real reasoning behind it went like this: "We will never, ever nominate heavy metal for anything. Crap, people keep complaining, what do to? Let's just create another category so these people will shut up. That way we don't pollute our other categories with this ilk."