That last sentence is the most important. The issue with margin debt, in particular, is that the unwinding of leverage is NOT at the investor’s discretion. It is at the discretion of the broker-dealers that extended that leverage in the first place. (In other words, if you don’t sell to cover, the broker-dealer will do it for you.) When lenders fear they may not be able to recoup their credit-lines, they force the borrower to either put in more cash or sell assets to cover the debt. The problem is that “margin calls” generally happen all at once as falling asset prices impact all lenders simultaneously.
