Just like with the recent crossing the maritime border with a patrol boat, test firing missiles into the Sea of Japan is another well known trick in the North Korean playbook:
North Korea has test-fired four short-range missiles into the sea, Seoul’s defence ministry said, in an apparent show of force to coincide with the South’s joint military exercises with the US.
A ministry spokesman said the missiles, with an estimated range of 200km (125 miles), were fired off the east coast of North Korea.
“Our military will maintain tight vigilance in preparation for additional launches or any military provocation from the North,” the spokesman said.
North Korea regularly carries out short-range missile tests, and has used them before to display its anger at the annual military exercises. Observers said the tests were unlikely to trigger a significant rise in military tensions. [The Guardian]
You can read more at the link, but the South Korean Defense Minister Kim Kwan-jin says the ROK military is prepared to respond to any North Korean provocation:
“The Kim Jong-un regime has continuously developed nuclear and missile programs, and increased conventional weapons,” Kim said in a congratulatory address. “It is very hard to predict what will come next as Kim Jong-un could hurriedly make a wrong judgment under the unstable political situation…Our military is prepared to counter any provocations.” [Yonhap]