We can't know for sure at this time. There was a wall of silence on social media (telegram,twitter) after the "hack". Site was down for a few hours. Now they have an announcement. There is a reddit thread about it:
https://www.reddit.com/r/ethtrader/comments/6nsyae/coindash_website_hacked_55_mil_gone_do_not_send/

It's possible it's a hack, but I can't rule out the team behind Coindash. Quite possible it was planned.