[JasonC]

Um, the banks are issuing various margin calls to financial institutions they lent money to, with various financial securities held as collateral for those loans. The margin calls occur precisely because the collateral the banks already hold is declining in price, and their lend-ees are obligated to put up more funds when that happens, to maintain a certain ratio of their own risked capital, to the amounts they have borrowed. The banks are not receiving such margin calls themselves; quite the contrary, they are both issuing them, and seizing collateral when those so called, can't come up with additional funds.

[AndyJackson]

Tell us all something else we didn’t know, oh supreme master of the universe.

[palmer]

Um, the banks are issuing various margin calls to financial institutions they lent money to, with various financial securities held as collateral for those loans.

The Fed is allowing financial institutions to post those "securities" as "collateral" so they can meet margin calls using the T-Bills borrowed from the Fed. Otherwise there's no point in doing it.