Even a healthy bank had a run they would not survive, because fundamentally banks borrow short (deposits) to lend long (loans, bonds, etc). The old Bagehot quo

Even a healthy bank had a run they would not survive, because fundamentally banks borrow short (deposits) to lend long (loans, bonds, etc). The old Bagehot quote comes to mind: "Every banker knows that if he has to prove that he is worthy of credit, however good may be his arguments, in fact his credit is gone..." Anything that would cause depositors to worry and demand their money back quicker than the banks' investments mature would run into this problem.