Local North Korean petty officials or gangsters (the same in most communist regimes) out looking to supplement their income.
Drug running or other illegal criminal transactions are also done in the middle of this part of sea. Japanese crime syndicates used to(maybe still do) meet up N. Koreans there to get Meth made in N. Korea. Other kind of smuggling deals can also happen there. Russian yacht could be at the wrong place at the wrong time to witness such transactions. Or Russians may be also in it, for example, smuggling drugs into Busan, and they have some falling-out with their N. Korean partners.