I''m surprised that the other non-Korean markets are not following. If I could sell Steem for $6 on a Korean exchange, then buy it back for $1.3 on another exchange, then I would do that all day long (and get rich in the process). Why are no one doing this?
Sometimes the wallets are under maintenance and people can't deposit or withdraw for a certain time. You can't really know this information until you log into both accounts on both exchanges. What is most impressive to me is the inside Upbit arbitrage at that time mentioned by @kingscrown, but again maybe there is some info we are missing that we would have found out about if we would then have been inside this exchange.
Apparently it is not possible to withdraw Steem on Bithump right now. To me it looks similar to the Bytecoin pump a few months ago. link to reddit story about it