They had/have a major debt repayment in February. They have not been able to secure sufficient financing to roll that debt.
It may also be related to the discovery of the tax "issue" and the potential need to restate financials, the financing that they HAD lined up might have been withdrawn. They hadn't been able to secure the final $600m they needed. If they were back to "square one" on the whole $1.2b they might have figured that the game was over.
Theoretically BK could have been deferred until February when the $1.2b loan was due to mature and there was insufficient cash to make the payment. From a cash flow standpoint, there was no NEED for the company to file now, unless, of course, they were in violation of a loan covenant and the lender called their loan.