"I declare martial law to protect the free Republic of Korea from the threat of North Korean communist forces, to eradicate the despicable pro-North Korean anti-state forces that are plundering the freedom and happiness of our people, and to protect the free constitutional order," Mr Yoon said.
Majority party, which is in opposition to him, was about to impeach members of his cabinet, apparently. And he isn't very popular with the people. Also, it's been around a year of talks about impeachment for Yoon Sok Yeol himself.
Must be commie spies, clearly.