The EtherDelta domain was hijacked
The hacker cloned the site, hacked the DNS, and pointed the cloned site to a malicious address. People then had no idea they were using the bad address and funds were stolen.
"Dear users, we have reason to believe that there had been malicious attacks that temporarily gained access to @etherdelta http://EtherDelta.com DNS server. We are investigating this issue right now - in the meantime please DONOT use the current site."