Citadel Securities' head of equity and equity derivatives strategy Scott Rubner wrote to clients on Monday that the bullish setup that carried the S&P 500 to records in August is changing. He noted that September is historically the worst month for stocks, and options prices are now the cheapest all year, making the risk-reward of buying protection compelling. Rubner also cited slowing corporate buybacks and increased macro events as reasons for near-term caution.