Market makers are challenged in fast markets—when either buyers or sellers are dominating and prices are moving rapidly. When this happens market makers are obligated to continue quoting bid and ask prices that maintain some semblance of an orderly market. If they start accumulating uncomfortably large net long or short inventories they may start hedging their positions to protect themselves. For example, if they are short Bitcoin futures they can buy Bitcoin futures with different expirations or directly buy Bitcoins to hedge their positions. The hedged portion of the market maker’s portfolio is not sensitive to Bitcoin price movements—their profit/losses on the short side are offset by their long positions.

If anything, the problem seems to start with incredibly lax risk management at this exchange.  According to the OKEX statement, the risk management team 'immediately' contacted the client to reduce the size of the trade - begging the question - how did their risk management system allow the trade to occur in the first place?  On the bright side, something like that should be easy to fix, but it is indicative, potentially of how many simple things are being overlooked in the rush to make money from crypto trading.
Note that you could just keep bitcoin on CryptoFacilities waiting to make the trade so you don't have to wait to move the bitcoin you bought over. This is called see-saw arbitrage model, where you keep funds on both exchanges to avoid having to wait. This is fine, but you can't ignore that there is extra capital being used in the play, so it affects your rate of return and capital utilisation. We will not use this method, we will do a full, complete, legitimate arbitrage process.