There's a total of 16.9 millions of Bitcoins in Market Cap. 400$ millions worth of BTC, which was between 7-9k USD in February, that's ~50k Bitcoins, which is only ~0,29% of all Market Cap, how could it cause 75% crash?
Because the market cap is just current price * number of bitcoins. It says nothing about how deep the market actually is. And since there are billions and billions of dollars worth of bitcoin out there mined for pennies, it probably isn't that deep.