Thanks for the link to the article @Ingram I also followed the link in the article to the Twitter thread which provided further details. It was very convincing and the images used look legitimate too. Often the lines drawn over a chart to prove a point don't match the underlying data points, but this one looks fine.
Unfortunately there is a big price difference between any cryptocurrency, and what the average owner believes it is worth. That's because the vast majority of owners don't have their coin on the market for the price they think it is worth. The market is driven by supply and demand and when two traders agree on a price, that amount, and to some extent their trade volume, sets the price of the cryptocurrency.
In a fair market, the amount being bought and the amount being sold would hover around equal as most people believe it to be worth similar amounts. But in a manipulated market, someone with a very large amount of coin can put it all on the market at once. The market will quickly run out of buyers with cash or liquidity enough to buy what's being offered. So the seller has to drop their price to lure out more buyers.
If the market were for something of accepted value like oil or rice, the owners are not too concerned about price fluctuations because they are very confident that the price will return to normal quickly. But with cryptocurrency, people are often very unsure why it has any value at all and are afraid that at any moment it may become worth nothing. This state of fear creates a situation where if the price of a coin drops suddenly, many owners will try and sell as soon as possible, before the price drops too low.
To sell quickly, you have to offer a noticeably lower price than the other sellers. This business of undercutting the other sellers causes the price to drop very quickly, and this continuing sharp drop makes even more people afraid and want to sell.
But large holders, wanting to sell a lot of coin know this. So as soon as the price drops, because they know the drop is artificial, they are not afraid. But they also don't want to sell their coin for less than they have to. From the thread that this article is based on, it looks like they have done a very clever thing. Just after the price has dropped, but before it has gone too far, they boost it back up again by putting a lot of buy orders on the market at a good price. Suddenly the sellers realise the drop was only temporary. And some will even take the opportunity of the small drop in price to buy more coin. This pushes the price back up towards where it was before the dump. Then the large holders can have a second bight of the apple and dump more coins at a good price. The second dump then runs away with falling prices as people panic sell and nobody tries to correct it.
The pump side of the Pump and Dump process is just paying some people to hype the coin. It is possible that they might also put in high priced buy orders to help that along at the same time. But because the amount required to affect the price is much less than its percentage of the market cap, this can be very effective.
As for speculation that for Bitcoin this was caused by futures-market traders, that is very hard to prove. It is true that these traders would have been purchasing some coin to offset their risks. But it seems unlikely to me that they would have done that before they got a lot of nods from regulatory approvers. And that didn't come until fairly late into 2017 when the price was already quite high. So for them to be selling for a profit during the final dump seems unlikely to me.
It is equally as possible that one or more of the bitcoin whales out there chose the date because it was a date when there was certain to be a lot of interest in Bitcoin, keeping the price higher as they began to sell. They may have even hoped that a big news day like that might drown out noise of a dump for a bit longer and make the market slower to react. But that is all speculation.
Finally, a large part of the correction could be natural. Bitcoin experienced an extreme spike in value that year. Previously, predictions that it would breach the US$1,000 mark were met with laughter and scepticism. So to see the correction keep the value of the coin so high above its long-term average makes it look like Bitcoin has recognised value, but not as high as the speculators in late 2017 had hoped.