Chinese investors’ enthusiasm is helping China’s dollar bonds trade at lower yields than U.S. Treasuries—a historical anomaly because the U.S. securities have historically been considered the safest of investments.

"August lulled investors into tight spreads combined with ever-strong demand for credit. Issuers can borrow at rates we haven't seen in a couple of years."

As the Japanese central bank moves to cap sovereign bond yields, investors will seek higher-yielding corporate bonds.