OKEx, Huobi and Binance was moving tons of crypto's like every 3 hours so unlikely that this is a hacked, just the usual thingy top exchanges are doing today. Shifting their funds to wallets that are secure and safe. Nothing to be alarm here.
Shifting their funds to wallets that are safe is obviously a good thing, but the three exchanges you stated do that so frequently, that I am not so comfortable with it. If you look at Bitstamp's or Coinbase's cold wallets, their funds aren't moving at all unless they have to fill up their hot wallets. There is no reason as an exchange to keep moving a large chunk of your cold funds back and forth so frequently.
There is a suggestion that this could be a Bakkt wallet:
It's funny how people suddenly link every single high value transaction that happened in the last couple of days to Bakkt's custodial service. They wish that many coins would be handed over to them to take care of. This is crazy FREE exposure for Bakkt that otherwise would have cost them tens of thousands of dollars in marketing.