They had $21.9bn liabilities in Q3 2017, compared to $489m in 2011. Couldn't find 2008 numbers but sure that debt was much lower then. Bankruptcy doesn't just mean having low cash levels, it means not being able to serve debt in time. That risk has increased meaningfully with increases in debt. Missing the coupon on one of their bonds by just a few days would put them in technical default and could trigger bankruptcy.