One of the supposed benefits of holding cryptocurrency that has been peddled is that it is a strong hedge against a down turn in traditional securities. Here we have a new, real data point to the contrary.
To me it seems much more likely the large exchanges just dumped in order to make a killing off the leveraged longs.
So in short people did actually believe in it as a shore of wealth and exchanges manipulated this belief in order to selfishly profit.
This isn’t something that was easy to predict, my comment is obviously a reflection of what happened today but I think it’s the most logical explanation.
And other cryptocurrencies are still very much tied to BTC, so when BTCUSD falls dramatically, they do as well.