Also, your talk about viewing the order book explains why someone could buy more than 50,000 shares without moving the price (the order book showed there were sellers at that price for that quantity). I imagine big shorts could also use the order book to predict the opposite effect (big price change on a big sale in certain circumstances) Much thanks.
Another way of doing that order may be "fill or kill", particularly once they see the liquidity there. Makes sure no one has a chance to react to it before it's filled, else kill it w/o displaying.
For more info on order types ("dark" order types shown on pages 4 & 5; full list pg 14-28) - https://www.virtu.com/uploads/documents/KCG_Demystifying-Order-Types_092414.pdf
My current assumption is the times we see price "collapse" down suddenly and instantly, that (in bold) is what's happening. Not always "shorts" though, can also just be algos designed to target that kind of thing, given the obvious profitability.