When the MPAA ratings were introduced in the 1960s, X was used as the highest rating, equivalent to the way R is used today. Porn producers in the 1970s picked up use of the X-rated banner, and using XXX came about as a play on that, indicating that they are more extreme than X-rated. The MPAA soon dropped usage of the X rating to avoid the association with porn. That's when it picked up NC-17.

It's mentioned in this article. Perhaps someone else can furnish you with a more in-depth source, but that's the gist of it.