The problem is, a DDOS attack doesnt cause their entire server to crash. It only shuts out most users but there are some lucky ones who keep refreshing still managed to log in. And when this happened, it will cause a panic and those who managed to log in will start selling fast hence the domino effect kicks in (triggering margin calls etc).
I have experienced this once at Poloniex during the massive buying of Ripple last month. Have stopped trading at Poloniex since.
Then why the price fall in the first place to cause the panic? I think those who manage to start the DDOS should remove a lot of buying offers from the list right before the start of attack and spend a good amount of BTC in order to make the price fall.
The panic was caused by the DDOS attack on the server and people not able to log in, not by the fall in price. The DDOS attack most probably initiated by big whales with programmed dumping to cause a chain reacted sell down so that they can accumulate again at much lower price
Anyways, these are just my opinion. Nobody really knows whats going on unless you are one of those who are doing this.
A website not responding doesn't cause a panic for users to sell their coins IMO, but your idea about whales doing a programmed dump is correct as that's the only way for me to cause a panic sell like that.
Thanks for sharing your knowledge.
It is even happening right now I think. Poloniex is out of reach and many prices are falling down including XRP, STR and XEM