A lot of people that trade manually (not with bots) put in stop losses,. They do this to protect themselves should the price fall below a certain threshold. These seem to get automatically triggered when these flash crash occur on crypto exchanges.
Worse still if you say you put in a stop loss $300. During ordinary trading you can expect, should it get triggered to execute a trade in the $295 - $300 range (depending on the liquidity and size of the order).
However during a flash crash, by the time your order gets executed you could end up selling your coins at $50 (or in the case of the GDAX incident 50 cents). Which would not be your intention. Particularly if the price goes back up to normal range say $350 minutes later. People lose a lot of money on an isolated occurrence that happens on a single exchange that is out-of-step with the rest of the exchanges.
Someone with leveraged cash and visibility of stop losses could use that money and information to crash the market and pick up cheap coins as the stop losses are executed, if the exchanges do not have the necessary safeguards in place.
Add to that, that the exchange has visibility of everybody's stop losses and it is largely unregulated space.... it's an open invitation for consumer protection regulators to stick their noses in.
RE: Anyone else notice the Bitcoin Cash 'flash crash' on Bitfinex? Regulators will.