Check your form in front of a mirror. When Squatting heavy, sometimes your form will falter and your body will shift more of the weight onto your dominant leg. When you put yourself under a heavy weight, your body doesn't understand form; it only knows it has to get that weight up however it can or it will be crushed. Thus, it will play to its strengths.
I had a similar problem when I started Squatting. Would shift the weight to my left leg by accident. Took a few weeks to get the form right again.