My understanding is as follows: A lot of corporate debt is backed by the real estate. For example, McDonald's food operations are far less valuable than its real estate portfolio. If that property is now worthless because no one wants it and it's unoccupied, banks now have assets worth less than what's owed on them. That in turn means when the loan term ends, banks can't just re-finance the debt, because the collateral that secured the loan in the first place isn't worth what the debt is. That means big problems for companies who now need those loans as a source of cash to pay off the old loans. They now have to scrape up actual cash to pay, leading to more austerity. Because corps can't pay the banks, the banks lose out on revenue, which means they have to tighten their belts, and so on and so on in a self-reinforcing spiral. If the corps default, the banks can seize the assets, but again, they're worthless, so it's a one-two punch.

It's a giant shell game, and from what I've read economists are afraid a 2008-style crash may be in the works due to the cycle of debt above.