This is from an article pertaining to the Russian stock market (halted, 2 days in a row now), but it may very well happen here in the U.S.

“Heightened counterparty risk means that the only place to raise cash is the equity market,” said Julian Rimmer, head of sales trading at UralSib Financial Corp. in London.

“Every time the market opens we have selling to meet margin calls, which triggers stop-losses, more margin calls and redemptions.”