Back then it was the "Grand Prix" series, instead of the Masters Cup series today. He didn't want to sign because he knew he could have more fun outside the court. Many say Vitas G. introduced him to the lifestyle. His drug use brought out his psychiatric instabilty or simply fried his brain.
Mac's book touches a bit on the retirement, but a `91 Sports Ill. article along with a book from former business partner Lars Skarke gives a great understanding to Borg's mental illness. The most wretched parts are about Borg's "Cave" in Stockholm during the late 80's. I can't describe it in any way TW Boards would approve.