It looks like this is the response to North Korea’s ICBM launch, holding a trilateral missile defense exercise:
South Korea, the United States and Japan held a joint naval drill, the South Korean Navy said Sunday, in a show of strengthened trilateral military cooperation against North Korea’s escalating nuclear threats.
Korea Times
The three allies staged a missile defense exercise in international waters between South Korea and Japan, Sunday, mobilizing three Aegis-equipped destroyers ― ROKS Yulgok Yi I, USS John Finn and JS Maya ― to enhance readiness against North Korea’s missile provocations.
The previous joint drill was held on April 17 and the latest one was the fourth combined exercise to take place during the Yoon Suk Yeol administration.
You can read more at the link.