Plummeting oil prices and speculation about how the Federal Reserve's plan to tighten monetary policy would affect corporate borrowing costs has made companies more vulnerable, Vazza wrote.
"The current crop of US speculative-grade issuers appears fragile, and particularly susceptible to any sudden, or unanticipated shock," she wrote.
Arch Coal was the most recent addition to the list, having its credit rating downgraded to "speculative default" by Standard & Poor's last week after the coal producer missed about US$90 million in interest payments and exercised a 30-day grace period with the holders of some of its notes.
Looking ahead, S&P expects the US corporate default rate will rise to 3.3 per cent by September 2016 from 2.5 per cent a year earlier. The bulk of the failures will come from companies in the oil and gas sector, which accounted for about a quarter of this year's defaults.