Is it merely a scheduled maintenance as the trading platform claims or is it a hack?
Usually when an exchange gets hacked, their immediate response is "maintenance." Then they come clean hours later. Whenever this happens, in the back of my mind I'm always waiting for them to break the bad news.
That's exactly why everyone freaked out -- because Whale Alert reported the movement. 24,000 BTC is a pretty huge spend for an exchange like OKEx. With exchanges getting hacked left and right, it's no wonder people are on edge.