Underneath what you are looking at is the SPY starting the Monday after options expiration through the next ten days. Every month has shown some weakness after expiration and the context of every month has been the same, a rally up to expiration. August was the only month that we did not see a rally leading up to options expiration.
Opex Front Run Rallies h/t NorthmanTrader
10-day closing trough after expiration.
Visual of the 10 days after expiration.