It's the fed selling back the treasuries they bought from everyone during the pandemic. The theory is that if a bank's "cash" can be replaced with treasuries again, they're draining liquidity from the system. However, that money the bank received after selling their bonds to the fed in the pandemic is probably invested somewhere else (loans) and not just sitting in cash. So whoever is buying up those bonds from the fed repo has to sell something to free up the cash to buy.
If you look back on the SPX chart, there was a notable correction in February 2016 when the fed hit a previous record in their repurchases. There was actual talk of rising rates at the time too, and those did begin to step up in November 2016.
Basically, it's a tough way to time the market and might be more useful in hindsight, but the chart will show exactly how much money is being "drained" from the system by federal reserve operations. Looks like they did around $10B today.
