On S. Korea's side, there are left-wing (actually pro-North, too,) groups who also oppose this agreement. For starters, they won't accept anything which will make the current government look good. Furthermore, anti-Japanese stance is their favorite repertoire to legitimize their existence. They use it to paint their political opponents as the successors of Japanese collaborators. They are out to deny the entire history of S. Korea as Japanese collaborators oppressing Korean people. They have to get the flame alive. There are legitimate issues to be discussed, but they use this emotional topic to whip up the rage. All these civic groups are infiltrated by the radical left. They needlessly exacerbate the situation to serve their own interest.
Up until the current agreement, Japanese were to blame for not fully addressing this problem. The tension lasted for at least three years, which made U.S. worry that the crucial alliance of S. Korea-Japan-U.S. may be at risk. Now that Japan made good faith efforts to resolve this issue, this has to be wrapped up and three countries need to move forward.
However, Japan's pro-militarist elements and S. Korea's pro-North elements are working together to undermine this, because it serves the interests of both. For now, both S. Korea and Japanese government are standing by their decision. Hopefully, it will stay that way. If any of them starts to renege or reinterpret the agreement to basically water it down, it will create a mess, which cannot be cleaned up for quite a while.
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