The only reason the ask side would stack is because people are selling causing the bid side to diminish and the ask side to grow, or, people are lined up on the bid side trying to buy, which also causes the bid side to diminish, and the ask side to get larger. We are not seeing any selling so it can only be the former.
The reason the bid side is diminishing is because the MM's are concentrating on where the pressure is. In this case, the pressure is on the bid side to buy. Which by the way, also causes the pps to go backwards.
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