The body is moving in multiple directions in Warrior II, so much that one focus of the pose as a hip opener can get lost in the mix. Left and right fingertips moving wide apart from each other; spine and head moving toward the sky; front knee extending and bending; rear leg lengthening, etc.
To get the hips open, teachers give such cues as “bring your front thigh parallel to the ground” or “put some weight on the outside of your back foot” or the classic, “tuck your front hip under your pelvis.” These cues can lead to focusing on one hip or the other (which of course may be appropriate depending on the circumstances). So, for example, when giving the cue “bend your front leg toward 90°” I can see everyone move forward and lean forward as they shift their focus to the front leg.
One visualization cue/technique I’ve been giving in class to get both hips opening equally and at the same time, while keeping the spine long and tall, is to “imagine your pelvis sinking straight down toward the mat/Earth.” When I give this cue I can see the entire class sink straight down, rather than forward, as both hips open and the rest of the body keeps good form. It’s neat to see. Give it a try in your class or your practice.