He was mad with Shueisha and took Shaman King with him to Kodasha, if I recall well
Well, they basically made him finish his series in a rush and ended up writing an apology message on SK final chapter iirc. Then in the Kanzenban Edition he was able to modify the series ending and make it a bit better than the one that was published while serialization and in the original volumes, what he called a
true ending. Then they gave him the chance to continue in Jump X publishing Shaman King Flowers, but this sequel was also axed along with the magazine suspension October 2014.
In the end, he was actually never able to finish the main series correctly or continue with the sequels he had planned, so he just took everything and went to Kodansha, where they started to release a new Shaman King edition in 2018 and where he publishes Shaman King: The Super Star, with a quarterly schedule iirc.
EDIT: Found those afterword messages.
This one was after chapter 286, when the series was axed on the last original Volume 32.
And this one was the final afterword for Shaman King after chapter 300 included in the Kanzenban Edition, Volume 27.