Radio Buttons Keep Changing

For reasons unknown to me, when I’m writing a new post, the “radio buttons” (Bold, Ital, Left Justify, etc..) sometimes appear as text (del, ins, img…) and the editing shows HTML code, not WYSIWYG. I’m looking for the button to change back to WYSIWYG editing, but can’t find it. How do I change this back? It seems to change by itself. Not the first time, either.