Liquidity characteristics of what banks are holding. Massive buying of bonds yesterday, not seen since 2008 he said. Because cash isn't fungible in a way that T-bills are. When you're borrowing against assets you want those assets to be as close to risk-free as possible, otherwise the lender doesn't want to.
In the big global economy there isn't enough collateral to go around.
A hedge fund chasing yield doesn't want T-bills. They want EM high risk, but to buy them they using the repo market, but you can't go to the repo market with these highrisk junk bonds assets to get a loan. You won't get the rates you want to get. You need something really secure. So you go to a money dealer and swap your junk bonds for the T-bills of someone he knows (an insurance company or pension fund). They lend them out for a small spread. So you're already multiplying collateral. A single T-bill might be securing multiples of these financing trades. "These re-use and re-pledge chains."
Then look at derivatives, which nowadays may be more likely even than the repo markets as the cause of the problem.
More and more, financial participants are being forced to do the same things at the same times because of the systemic conditions. Now, pay high levels of premiums for T-bills when there's problems in the collateral marketplace.