It’s a fairly complex question to answer in short form, but here are some of the most common reasons.

1. Cuing the exercises – ensure you are mentally lining up to use both sides equally, it might be worthwhile doing some sets in barefoot to feel the weight distribution through the floor. 
2. Opposite side ankle mobility restriction – this is probably one of the most common reasons to shift 
3. Hip mobility – this could potentially be an issue, especially if there is a large difference from left to right 
4. Hip stability – an imbalance instability on one side could cause you to shift your load across to the opposite hip.
5. Hip structure – we are not always symmetrical. You could have a hip structure, which means you cannot stay level through the squat. This could be the overall structure like retro or anti-inverted hip or more functional with the presence of a Femoroacetabular Impingement – FAI (cam or pincer).
6. Not creating lateral tension into the floor – the squat is not just about knees forward and hips back. How you apply left to right (lateral) tension into the floor helps create a more level squat pattern.

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