The Epicurean Dealmaker talks about how leverage affects apparently unrelated markets leading to a contagion.

Well, consider this. A fund with a highly levered balance sheet, and its investment fingers in many pies, is hit with losses in one of its sub-portfolios. Due to the nasty two-edged bite of leverage, its equity drops significantly, and the only way it can restore its risk profile is to raise more equity or liquidate some of its investments. Given the poor market conditions in the affected sub-portfolio, it is often more prudent to liquidate securities in other sub-portfolios. But this, as you can imagine, puts downward price pressure on securities in those previously unrelated markets. Presto, contagion. This is the “common holder” problem which some believe is the primary culprit.
 
Consider further. What if a substantial portion of our hedge fund’s holdings consisted of loans to other investors—hedge funds, perhaps—whose own portfolios were experiencing losses? Well, then, “liquidating” those positions and reducing its risk exposure would look an awful lot like calling the loans, or reducing their outstanding balances. Finally, add this to the mix. What if our fund had another side to its business, which generated revenues from the origination, market-making, and placement of securities, which revenues were negatively affected by turmoil in some or all of the markets where it also had investments? Well, that would be a triple whammy, and our little hedge fund would look an awful lot like a prime broker investment bank.