There are claims being made that the Trump administration is pressuring the Blue House to buy F-35B’s:
Shortly after the Korea-U.S. presidential summit in April, the Blue House asked the military to consider the possible procurement of F-35Bs, a more expensive version of the stealth fighter jets Korea earlier purchased, an opposition lawmaker told the JoongAng Ilbo on Wednesday.
Rep. Baek Seung-joo of the Liberty Korea Party, who once served as vice defense minister, said the National Security Office of the Blue House discussed the purchase plan with an Air Force official on April 18. The summit between President Moon Jae-in and U.S. President Donald Trump took place on April 11 in Washington.
In 2014, the Air Force finalized the first phase of a next-generation fighter jet project and confirmed the purchase of 40 F-35As, the most common version of the fighter jet built by Lockheed Martin. The deliveries are scheduled to be completed in 2021.
The second phase of the project will start in 2021. The military is to spend a total of 4 trillion won ($3.35 billion) for five years to purchase 20 additional stealth fighter jets. Speculation was high in the military that Korea would buy more F-35As.
Although F-35Bs have short takeoff and vertical landing features, which would allow deployment on a light aircraft carrier Korea is planning to build starting next year, military sources said the Air Force prefers F35-As. They said F-35Bs have limited weapons capabilities, operational scope and mobility compared to F-35As.
Joong Ang Ilbo
You can read more at the link, but I think the fact that the Japanese announced in August they were buying F-35B’s to put on their helicopter carriers is the more likely reason. This could just simply be a case of keeping up with the Jones’s.