A UBS Group AG analysis revealed that the speculative-grade debt market is starting to show some cracks. Investors are less willing to finance the lowest-rated companies, with borrowers such as Cliffs Natural Resources Inc. and Capital Product Partners LP struggling to attract lenders. Relative yields for the riskiest portion of the market have been increasing, and it's getting harder to trade the notes, according to UBS's analysis.

Historically, this sort of backdrop has signaled trouble ahead for stocks. During downturns, junk-bond prices tend to start falling three months ahead of equities, data compiled by the Swiss bank show.

"There are clear signs that we may be approaching a turning point in the credit cycle," UBS analysts Ramin Nakisa, Stephen Caprio, and Matthew Mish wrote in a report Wednesday. Given corporate-debt markets have swelled way beyond their size before the 2008 financial crisis, stock pickers need to pay even more attention to what's happening in bonds this time around, they said.