I agree with Julia. Holding the crop between your hands works for this habit, sometimes putting it across your back and holding it in place with the crook of your elbows helps too.
Ah mine does this too and for me it's just a habit I have to keep working on. I find it's normally because I'm leaning and dropping my shoulder to the left. But really you just have to stay super concentrated on it and it's a crappy habit to break
If nothing else works, jus tie A knot in your left rain where it keeps slipping down to and keep your hands above the knot…
Try an equicube it makes your whole body position overall better. I borrowed my trainers and its awesome
Ride with a crop in between both hands and it helps learn to keep hands even and up