2017.3のバグみたいです
報告多数ですね
https://forum.unity.com/threads/very-irritating-ui-bug-has-anyone-else-experienced-this.510596/

I encountered this after upgrading to 2017.3. One of my UI windows was all messed up,
so I tried to fix it, and upon hitting "Apply", some elements auto-centered.
After reviewing everything for rogue layouts or anything like that, and with it happening on other elements, I went to the internet and came across a few threads like this.
Fortunately, I saved a copy of my 2017.2 project and going back was as easy as renaming a folder.
But I think this is a pretty severe bug, it could effectively stale a project relying heavily on UI.