It is due to the delay of a mob while turning, The mob will be fully turned before you see the animation, roughtly ¼ to ½ a second, example, you see titan starting to turn for landslide, he is already fully turned and picked his spot to hit, you just don't see it yet. You just have to anticipate these movements. When you activate a WS it will register where you pushed it even if you move, but mobs can already have moved too without showing it yet due to latency.