It happens as you lunge fwd.
I find most people try to put their leading leg straight in line with the back one. Step a bit to the side as you go fwd. Aslo, unless you are comfortable with regular lunges, there is no reason to do rotational lunges. It's a progression thing. If you want to improve your form go back and do in progression:
static lunges
reverse lunges
forward lunges
walking lunges
If you can have someone experienced check out your form or get things checked by a PT or a coach, do that
