Weight on outside of one foot

I've recently noticed that on "heavy" squats, I don't feel like the weight is evenly distributed across my right foot - instead, I find myself with a lot of weight on the outside of it. I have tried experimenting with stance width but that doesn't seem to change the fact.

At lighter weights I can focus and keep my weight more centered, but when fatigue hits I also revert to that uneven distribution.

Any ideas what sort of imbalance could be causing this, and what I can do to improve it?