New York Times: In a Credit Crisis, Large Mortgages Grow Costly

At the heart of the contagion problem is the combination of complexity and leverage. The securities that financed the rapid expansion of mortgage lending were hard to understand, and some of those who owned them had borrowed so much that even a small drop in value put pressure on them to raise cash.
“You find surprising linkages that you never would have expected,”
“What matters is who owns what, who is under pressure to sell, and what else do they own,” he said. People with mortgage securities found they could not sell them, and so they sold other things. “If you can’t sell what you want to sell,” he said, “you sell what you can sell.”