I read somewhere that back as far as 2005, a penny loss in FNM stock price would cause Citicorp. to lose about 600k, how that works is a bit convoluted but has to do with the credit default swaps, credit lines, and crdit downgrades, the stock price was a symptom not the cause.
The stock price then was 70, now its a buck and change. The investment banks were leveraged anywhere 30 to 70 times.
They collectively now owe more money than exists, probably, so its easy to understand the late unpleasantness.
Much, much more.
We haven't even discussed the REST of the derivatives market.