from Mr Top Step
The reason the S&P futures sold off yesterday is because all the midmonth rebalancing that helped push stock higher over the last few days forced the S&P options players (funds) to pay up / roll higher. Whether the short selling option funds rolled higher, bought futures to protect or liquidated, all of that added up to buying pressure. After two solid weeks of increased volume in the S&P, the volume had let up; to us that meant all the rolling / buying had been used up, leaving the S&P susceptible to a selloff, and that is exactly what happened. With few bid and sell stops under 1651, the algos and the sell programs kicked in and took advantage of the low liquidity environment.
https://www.mr-topstep.com/index.ph...-and-p-late-day-selloff-why-and-what-it-means