What are you talking about? You know trades go back and forth... Those are not some new coins that didn't already exist on the exchanges that just gets withdrawn from the exchanges on one go... Exchanges don't even have such amount of fiat to handle.
That mistake was made in the famous "flash crash" of 2010. Trading volume appeared to be high, but it was the same money going round and round in HFT transactions which were a net wash. When someone did a real sell for cash, prices crashed.