This makes no sense to me. (I understand it, but why it happened)
A seller wants the best price possible. If you start hammering the sell orders, you move price too much, so you end up with way worse fills, as we see. Some very lucky buyers scooped up bitcoin for incredible deals. Its also interesting that all of this could be done with only 400 bitcoins. Was the seller so desperate to sell in the next few minutes? Were they testing liquidity for future games? Did stops get hit and somehow, the seller was subsequently also buying trying to load up at better prices?
I have worried that as ETFs hide away more and more bitcoin, there is less circulating, and since price is set at the margins, the extremes of price swings could get worse. Of course this also works in reverse, where a buyer can easily push price up tens of thousands of dollars, and this is something I expect to happen in the future.
Most exchanges show their order book quite easily, but I've never studied it, and I have no idea if bigger exchanges could absorb this selling. I guess this is why OTC desks are important, and I have no idea why this seller wouldn't use one. He could have gotten a much better price I'm sure. There must be more to this story.