"The way I run our organisation is, I have high expectations and I demand a lot from my employees and Dan [Shamir] is the same way as a coach.
"There are certain people who are built for that, and who step up and want to be a part of that, and want to be a part of the next steps for the Breakers, and there's some people who aren't built for that.
"What Mike told me was that he had already sold his house and that he was moving back to Nelson.
"You can read into that, but if the house was already sold then you can imagine that this decision was not made today when we were on our way back from this NBA trip."
Fitchett was the last remaining member of the coaching staff before Walsh purchased the team in early 2018.
"I didn't intend to clean house, but here we are and I guess that I couldn't be happier with where our staff is and the direction our club is going."
The Breakers are wasting no time in finding Fitchett's replacement.
"We've started movement to get someone else in here, a high level coach who can help the organisation move forward, so expect somebody to be in Auckland to replace him by Monday or Tuesday at the latest," Walsh said.
"We've zoned in on two people."
The Breakers begin their ANBL season against the Sydney Kings in Sydney on Friday.