Well, it's a different turns some .
A possibility is an older dynasty, pissed off maybe by the acts of Imperial Japan's lords maybe, or early 'samurai japan's. Or older, complex dynasty fighting, rivality and dynastic links in between Yamato and Korean Kingdoms eras (it is speculated Yamato peoples had links with korean kingdoms).
Or problems around Ryukyu?
Or Yuan mongols.
But this need a China with a navy power... and this is an issue.