"Investors can buy aggressive deals at new issue, and they may be able to flip them for a day or two, but thereafter the secondary liquidity in the deal vanishes," he said. "The smaller deals that will inevitably be near-zero-liquidity issues generally come with a concession to more liquid deals, but that concession has dwindled to perhaps 25 basis points (bps) or even less in the high-yield market, and maybe 5 bps in the investment grade market."
The final emerging risk is a focus on "net debt," rather than gross debt or gross liabilities, Stracke said.
Read More Are junk bonds losing their 'high-yield' status?
"The consensus is that many companies are flush with cash, and therefore that cash can be netted against outstanding debt," he said. "But the devil is in the details."
He noted cash is often trapped in offshore tax havens, where repatriation is only possible after a hefty tax bill. In addition, analysts often use all of the cash on the balance sheet to calculate net debt, without considering how much may be needed for working capital to keep the business operating, he said.
Another concern is which debt figures are used as in bankruptcy, bondholders aren't the only people lining up to divide a company's recovery value, he noted, citing liabilities including pension funds, litigation reserves and deferred taxes.
Others also see rising risks in the credit market.
Read More Red flag waves over junk bonds as money pours into high yield
"Continuous injection of liquidity, hunt for yield and relatively better valuations have led to strong performance by corporate credit (investment grade and high yield) over the last five years. As a result, valuations appear increasingly stretched," Societe Generale said in a note last week.
It believes corporate credit is vulnerable if newsflow deteriorates.
"The acceleration of the re-leveraging of corporate balance sheets through buybacks and M&A might just be that trigger," it said.
—By CNBC.Com's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1