on both of the two interviews I've seen now, cnn and cnbc, he's mentioned the collateral requirement at clearing houses
in terms of the "liquidity problem" question by the cnbc tool:
my interpretation is that the level and/or direction of trading on these stocks was not expected within the normal course of business. so the broker had to temporarily suspend one side of the action since it didn't have cash on hand to support more buys
but that doesn't mean the firm isn't liquid. it may have plenty of money set aside for emergencies on short term money market instruments. but those funds may not be available on T0 (takes a day or 2 or 3 to clear into cash, depending on what type of short term product we are talking about)
doesn't mean the firm as a whole has liquidity issues. just means they were not set to handling this amount an/or direction of market activity