Because of pre-existing debt. When unexpected deflation occurs real interest rates on existing debt spike, and because of lower prices on goods and services businesses may not have the cash flow to pay their debts even though in a stable price environment they would be profitable.
When the banks that made the loans to the businesses are forced their losses on a default they in turn can default on their creditors, i.e. people who have made deposits and purchased bonds, creating a cycle of financial failure.