Bitmex is over leveraged and prices would have gone to near zero had they not conveniently shut down due to hardware issues. DDos... likely story.
"A large spread of around 16% was observed between BitMEX and Coinbase during the sell-off. This is likely because forced liquidations from leveraged perpetual swap futures tend to exaggerate the direction of any move. The market recovered after BitMEX went down for 23 minutes reportedly due to unscheduled maintenance. BitMEX recently published an update detailing two DDoS attacks."
Unscheduled maintenance during peak selling around 4k... mmm hmm. Not your keys... not your bitcoin.