When you move a normal page you have "Move associated talk page" autoticked.
When you move a talk page you have no option to auto move the associated normal page.
Moving talk pages happens often by accident when you actually wanted to move the normal page. After the move you have to think twice what happened now. Going back to to fix it is hard and even almost impossible for unexperienced users.
Also can't think of a "normal" situation wheny ou want to move a talk page without the associated normal page.
So please add an autoticked checkbox for "Move belonging <ns page> as well". Untick if you really want to move the talk page only.
Or unswitch it and auto move the associated normal page and make an unticked checkbox for "Only move the talk page".