Lehman's troubles arise from a familiar source. As this article in the Washington Post notes,
Lehman, the number one underwriter of mortgage-backed bonds last year, amassed a giant portfolio of properties and mortgage-related securities. But the value of the assets began to sink last year amid a spike in mortgage defaults by homeowners with subprime credit.One thing to look for now is what happens to the market for credit default swaps. Traders have suggested that even solid firms are finding it hard to buy such insurance now. This could be an interesting weak (note also Bank of America purchasing Merrill Lynch and the problems AIG seems to be facing).