There could be a lot of different reasons. Maybe her mom/ dad or dogs she grew up around did it this way too. Or maybe her vision is better on the left hand side? Or, it could be that she goes left when she is unfamiliar with someone or something.
Your dog circles to the left for the same reason a person walks more on their right or left leg, leans their head one way over the other, or writes with their left or right hand. It is just the natural direction for her.
