From the New York Times: "How Polarized Politics Led South Korea to a Plunge Into Martial Law" [1]
> From the start [...] Mr. Yoon faced two obstacles.
> The opposition Democratic Party held on to its majority in the National Assembly and then expanded it in parliamentary elections in April, making him the first South Korean leader in decades to never have a majority in Parliament. And then there were his own dismal approval ratings.
> Mr. Yoon’s toxic relationship with opposition lawmakers — and their vehement efforts to oppose him at every turn — paralyzed his pro-business agenda for two years, hindering his efforts to cut corporate taxes, overhaul the national pension system and address housing prices.
and also
> Opposition leaders warned that Mr. Yoon was taking South Korea onto the path of “dictatorship.” In turn, members of Mr. Yoon’s party called the opposition “criminals,” and voters on the right rallied against what they called “pro-North Korean communists.”
> (Mr. Yoon echoed that language on Tuesday in his declaration of martial law, saying he was issuing it “to protect a free South Korea from the North Korean communist forces, eliminate shameless pro-North Korean and anti-state forces.”)
So basically, Mr. Yoon was unable to pass his agenda (as his party never had control of the legislative branch), and rather than continue to negotiate, he decided to impose martial law, label the opposition communists, and then ban the National Assembly from gathering (they gathered anyway).
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[1] https://www.nytimes.com/2024/12/03/world/asia/south-korea-yo...