On September 6, Sam Goldfarb reported for the Wall Street Journal that the "average yield on double-B bonds, the highest-rated type of speculative-grade debt, has fallen to 4.07% from 4.69% at the start of May, according to Bloomberg Barclays data. The average yield on a tiple-C bond, close to the lowest-rated type of speculative-grade debt, has increased to 10.89% from 9.64%". Explain why the average yield on double-B bonds has decreased, while the average yield on triple-C bonds rose?