Typically what happens is that there's a stress event in the economy and the investment portfolio backing the liabilities has a mixture of fixed income and equities. The equities would lose their value by 15-20%, so the value of the assets would go down.
In addition, typically central banks move to lower interest rates. As you move the interest rates lower in a stress period when there's a recession, the liabilities go up. You get kind of whipsawed, such that the value of the assets goes down at that point in time and the liabilities go up.
Not every bankruptcy occurs when there's an economic recession or a troubling period, but many of them do, as a result of customers cutting back on purchases of their products.