You usually enter bankruptcy when you become insolvent, that is to say when you can't pay your bills (it can be the salaries of your employees, a bill from a supplier, debt interests, rent, utility bills, etc.). You can run a profitable business and go bankruptcy because you are out of cash, and nobody agrees to lend you some.

This is not unlikely when you have to pay your suppliers before you collect payments from your customers. And realistically that's the common case, not the exception. This is why cash flow is so important for any business.