From what I've been able to gather, there has been a decrease in demand for US Treasurys from certain segments of the market. This has resulted in primary dealers having to purchase the difference. They don't purchase USTs from cash on hand. Instead, they use the repo market to fund the purchase. However, the supply of repo market funds is relatively inelastic. Thus, you have a big spike in repo demand with a relatively fixed supply. This was causing the rate spikes we saw. Thus the Fed stepping in as lender of the last resort.