From an interview of Shimizu by Stan Pranin:

Why did you leave Hombu Dojo?

I couldn't get along with Tohei Sensei. I feel very sorry for O-Sensei, Doshu (Kisshomaru Sensei) and Osawa Sensei. I ignored their attempts to restrain me and left the dojo. I have been independent teaching Tendo-ryu now for six years. (Tendo is the name of my hometown). I am surprised that I have managed to come this far on my own having had to go through so many risky situations since I became independent. Although this happened many years ago, Osawa Sensei of the Aikikai told me that if I had stayed there I would have become a top teacher. I said that I am what I am now because I quit the Aikikai. If I still belonged it would be like crossing the sea in a heavily laden ship. I am just a small boat. When the wind blew, my boat listed and when it rained we had to bail out water. We were always exposed to risks. This gave me a certain amount of mental and physical training.