Similar to some others who have a deep straight arm catch, your head follows your spearing arm underwater. You can see how this compromizes not just your lateral (side-to-side) body position but also your horizontal (fore-to-aft) position. Your legs splay a little to compensate and your head, after it moves down, must move up to breathe.
In the article referenced by RadSwim, Coach Hines said imagine a dot on the top, middle of your head. If viewed from the front, that dot is currently moving all over the place. Try to swim with a "steady dot". mjm