I am an author, and I've had books published with two publishers. My first publisher closed due to health problems, but there are many reasons why authors change publishers.
Some change because there editor leaves and the new editor is not as enthused about the author's work. Sometimes, an author will change genres, and there publisher doesn't publish that genre so the author moves on. Then there are editors and authors who butt heads, which causes that editor to drop that author. If an editor thinks an author is getting stagnant she/he might get dropped. An author may be offered more money at another house, so she moves there. There are many reasons for leaving a publishing company.
I'm old school and have a tendency to be loyal to those who are loyal to me. After all, I worked for the same company for 36 years before retiring, but does that work in the publishing industry? There isn't much loyalty out there these days between companies and employees. Our young people tend to move a lot, and it's usually for the money.
Do you know of any other reasons an author would move to a different publisher than the ones I've mentioned?
Have a great Sunday, and I'll see you next week.