FYI OP claims $400mm USD was withdrawn to make a counterpoint about liquidity, but did not specify the exchange. Not sure why this would generate an argument? In addition, how can anyone infer that a sell indicates liquidity? A sell on an exchange only indicates one asset was exchanged for another.

FWIW I saw several large buys on the GDAX history today at the $18.4k price point. One was for 36 bitcoins, and I've never seen a purchase over 5. I shortly after saw another for 18.

is that why Bitcoin's value dropped something like 5%?

Which exchange?

source?