Pelvic Twisting on 1 Leg Squat

Hi.

I have been experiencing some pain in the pectinues area on my left side.

I have noticed that when I do 1 leg squats on my left leg my pelvis twists. As I go into the squat my left hip tends to go out slightly laterally and my right hip comes forward resulting in my pelvis twisting to the left. I try to control this when I do these squats but despite training for a while my body still wants to do this movement. This doesn't happen with a right leg squat.

Is there a particular muscle/s that might be weak on that side?

Thank you in advance to anyone that can help with my question. It is much appreciated.

Guess #1: your Pectineus is hurt so it doesn't want to work too hard; during single leg squat it would be working hard at mid-length to keep you in proper form, but if it went into further contraction it could still get the job done with less stress to itself - a Pectineus in full contraction would show extra hip flexion and adduction which sort of describes the 'twisting' your mention. Fix the muscle pain.
Guess #2: your gluts are weak; do hip hikes - right foot off a step, dip right hip then raise hip keeping left leg straight; do single leg squats assisting with arms until proper form is achieved (a cool way to do this is using a pull down machine, grab the bar and use the stack to 'unweight' you; lessen the weight as you improve) - :)

Thanks very much. I will do those exercises you advised.