Sold in 2012, the mortgage bonds have a higher concentration of loans to regional malls and shopping centers than similar securities issued since the financial crisis. And because of the way CMBS are structured, the BBB- and BB rated notes are the first to suffer losses when underlying loans go belly up.

“These malls are dying, and we see very limited prospect of a turnaround in performance,” according to a January report from Alder Hill, which began shorting the securities. “We expect 2017 to be a tipping point.”

Cracks have started to appear. Prices on the BBB- pool of CMBS have slumped from roughly 96 cents on the dollar in late January to 87.08 cents last week, index data compiled by Markit show.

That’s still far too high, according to Alder Hill. Many of the malls are anchored by the same struggling tenants, like Sears, J.C. Penney and Macy’s, and large-scale closures could be “disastrous” for the mortgage-backed securities. In the worst-case scenario, the BBB- tranche could incur losses of as much as 50 percent, while the BB portion might lose 70 percent.

Bearish Consensus

Alder Hill isn’t alone. Deutsche Bank, which famously bet against residential mortgage bonds in the run-up to the crisis, recommended buying credit protection on the BBB- tranche last month. So did Morgan Stanley.

There’s good reason to be pessimistic. After retailers had one of the worst Christmas-shopping seasons in memory, J.C. Penney said in February it plans to shutter up to 140 stores. That echoed Macy’s decision last year to close some 100 outlets and Sears’s move to shut about 150 locations. Delinquencies on retail loans have risen to 6.5 percent, a percentage point higher than CMBS as a whole, according to Wells Fargo.

Still, some aren’t completely convinced. Credit Suisse said last month non-CMBS specialists are helping drive the recent run-up in demand for credit protection. That raises concern too many people are chasing the same trade.

“The short feels crowded to us,” said Matthew Weinstein, principal at Axonic Capital, a hedge fund that specializes in structured products. “If these defaults start happening soon, the short will work, but if the defaults do not occur quickly, the first guy out could drive the market meaningfully higher.”

Cautionary Tale