Mito chose to wait until after the Kashiwa match to announce the departure of Kiyama as the team's leader following the end of the season.
He led Mito to be promotion contenders last year for half the season, and otherwise vastly improved the quality of play. But whether his fault or not, Mito is ready to finish in the lower half of the table as was historically the case.
Coincidentally, Shimizu's manager Kobayashi was also announced as finishing his tenure on the same day. This is likely just a coincidence, though. Kiyama would most likely go on to coach a team in the lower levels, a university squad as he did before, or perhaps even Kobe if they are relegated, as he previously successfully worked as part of that team's coaching staff.