Somebody placed a huge sell-order which crashed the price. This caused stop-loss and margin calls to be triggered, adding a huge amount of market order sells, which crashed the price to effectively zero (more sell orders than buy orders on the exchange).
Some lucky people (or the people who orchestrated the crash) snuck in some super low buy orders that got matched.
The price had recovered within a few minutes (once all the margin calls had executed). It just goes to show the risk of trading on margin
It seems likely that this was sharks taking advantage of some naive speculators. It wasn't even a lot of money for the finance world, and I bet they will made a tidy profit selling off all of that Ethereum they just bought.
One exchange had a big market sell order large enough to trigger enough margin calls and stop loss orders to create a cascade effect. It flashed to 10 cents for a few seconds. So the combination of volatility and careless traders created this situation as far as we know.