The New York Times explains Moody's downgrade of China's debt. China's credit growth has been exceptional, per Goldman Sachs in the "top 2%" of all credit expansions.
"China has accumulated its towering debt remarkably quickly. Goldman Sachs looked last year at how fast debt had accumulated relative to the size of the economy in 55 countries since 1960. It found that by the end of 2015, China was already in the top 2 percent of all credit expansions — and its debt shot up even higher last year. All of the other large expansions occurred in very small economies, some of which essentially lost control of their finances."