The only thing I can answer is, if the SHAREHOLDER is attempting to sell all his shares at a high PPS, and has it as an All or None transaction, it may never get processes, as no one will buy his amount of shares at one time.

I don't believe MM's have that problem. But, if I'm staring at millions of shares at the ask, and tens of thousands on the bid, my thought process is that more than likely, the PPS will drop to the bid at some point, as most people would rather buy small blocks at low price, rather than a massive block at the ASK. That is what people call "the wall"

That's just the way I see it.