In short, we could see a "tsunami" (the OP's word) of unforeseen losses in highly-rated commercial mortgage-backed securities (CMBSs), because credit-risk models used to construct and price CMBS tranches never considered the possibility that a large number of retailers would simultaneously break so many leases via bankruptcy, simultaneously impacting so many malls across the country.
Quoting from the OP:
> By seeking court protection, firms like Neiman Marcus Group Inc. and the parent company of Men's Wearhouse avoid the headache of protracted negotiations with individual landlords. But the moves threaten to upend huge swaths of the real estate market and the half-trillion dollar market for commercial mortgage-backed securities.
> "This is now black-letter law -- a debtor can cram down a landlord," said Melanie Cyganowski, a former bankruptcy judge who's now a partner at law firm Otterbourg PC. "If this becomes a tsunami of retailers rejecting their leases, it’s going to trigger another part of the sea change -- the mortgages held by the landlords."
Think: something analogous to the tsunami of unforeseen losses in residential mortgages that triggered a financial crisis in 2008, but with commercial mortgages this time around. It won't be pretty.