DDoS - Distributed Denial of Service
A coordinated attack on a system or network. Parties get together by inundating with requests for information or hammering it with malicious data. The goal is to disrupt the service that is normally provided. An example of this is if a webpage experienced a DDoS attack.
Cryptocurrency adds another element. When an exchange is hit, there is often a loss. This can be either from the actual stealing of money. Even if this is now achieved, customers of the exchange could be upset at the fact they were not able to trade while the attack was underway.
This timed with a market sell off could make some investors very unhappy.