Question: PG&E intends to file for bankruptcy on January 29th. They state it is largely due to the estimated $30 billion in liabilities from the recent disasters, fire suppression costs, and attorney fees.

How can they file for bankruptcy now, when they haven't even been found liable for anything yet?

Answer: 
They know the lawsuits and liability are coming. If they expect to have liabilities that will exceed their ability to pay and continue operating, they will have to file bankruptcy at some time.

Think about what happens if they wait to do this. How long do they wait? Say there's a few lawsuits that they are required to pay out. Do they pay them out? Does that mean the people who go to court first get 100% payout and the people who go to court later don't? That's not fair.

The responsible thing to do is to arrange bankruptcy protection as soon as you have to pay out money to someone who you don't think would get 100% of the money you owe them in a bankruptcy proceeding. Otherwise, you advantage the people who get paid first over those who would get paid later, and that's not good.
Companies (like pg&e) declare bankruptcy before actually incurring the anticipated debt from lawsuits. That way all new debt typically gets treated in the same class as “unsecured debt”. Senior secured debt typically gets repaid 100%. All other unsecured debt generally gets treated the same way, so if the company can only repay 80% of that unsecured debt, that debt is restructured as debt at a reasonable rate once emerging from bankruptcy. The remaining 20% of “unpayable” debt is converted into equity shares of the new company. Sometimes, the company can actually repay 100% of all debt if the debt’s interest rate is low enough. In this case, a portion of the equity shares in the old company can actually still be worth something. The uncertainty in PCG’s share price is grappling with this question right now. For example, PCG could possibly service an additional $15B (but probably not $30B) of lawsuit debt if it is restructured at a reasonable rate of, say, their long-dated WACC of 3.25%.