This is not specific to cryptography, it's a feature of all markets: if there's a scarce resource that a lot of people want, people will pay more for it. Parent flooded the market with whatever cryptocurrency instruments they held, suddenly making them a lot less scarce and so reducing their value. They could have avoided losing that by selling much more slowly (as they intended) or refusing to sell below a certain fixed price. In either case, they would probably not get quite as much as they had hoped for the whole lot due to a lesser version of the same effect.