Fair question! It’s primarily because there are a TON of calls that are ITM right now, thanks to the recent run. MMs are hedged long accordingly. On the other side of opex, once these options expire, there is no need to hedge anymore, so they will liquidate these long positions. The selling should put downward pressure. It’s similar to what happened in May too, but the effect lasted 3 days only.
By the way, the argument is inverted for the pre-opex period, where all that results in a bullish impulse, as detailed here at the beginning of the month: SPX Tech Anal: Will "Sell in May and go away" come true again, or should bears give it a rest? - #11 by The_Ni
Vol is low these days though, so the amount of hedging is not as much as it used to be, and so the impact of MMs is also not as pronounced.