Walton is who Vlade wanted 3 years ago, before he hired Joeger. I'm sure Vlade has been assessing Luke from afar ever since.
Joeger was a good coach, he helped this team develop an identity that worked for them this year. This identity of pace, I think, was always Vlade's vision. He designed this team and it's players together and built it to run. The first two years of Joeger's tenure was all grit and grind. Even the 2017-18 season was built around ZBo offense, which I'm sure Vlade signed as a favor for Joeger.
Vlade's vision to bring the Kings into championship contention centered around Fox, Buddy, Bagley and Giles. Vlade felt the Bagley and Giles should had played a bigger role this past year and Bagley should had started. Joeger didn't feel that way, because he didn't start Bagley all year (except for a few games). Whether Joeger's refusal to start Bagley was due to him feeling Bagley was not ready or if it was personal, I think Vlade may have felt it was the latter.
If Vlade felt that Joeger may not manage his team the way he envisioned, Vlade took the steps to ensure the Kings coach matched his vision for the team. I'm sure he spoke with Walton about his vision of how he would play the Kings roster and the way that Walton plays with pace, I'm sure his vision is aligned with Vlade. Walton also comes in with no possible personal agenda against the Kings prized big man, Bagley.
For the Kings to grow and for the players to be happy and want to re-sign with the Kings when the time comes, Vlade couldn't risk alienating some of his core players going forward. He decided to drop the hammer now, when his #1 candidate to be the Kings coach all along was available. Why screw around for show, you might lose Walton to another team.