1. leverage: firms get cash from broker and in return, brokers require posting some collateral (assets). this collateral value is constantly MTM by the broker. if this values falls, the borker will require variataion margin (additional payment) to keep the total amount held above the loan value. if firms do not have enough cash on hand, it will be forced to liquidate some of its other assets.
2. changes in collateral requirements: brokers can increase the haircut when markets are more volatile, creating extra demands on cash
3. mismatches of payment timing: must make payment on hedged postion, but you haven't received offsetting hedge payment yet.