The illusion is created because the figure is rendered side-on with no perspective. Because there's no indication as to which direction is correct, your brain j

The illusion is created because the figure is rendered side-on with no perspective. Because there's no indication as to which direction is correct, your brain just picks a direction and that's how you perceive her moving. Once your brain becomes fixated on one direction, it's hard to change your interpretation. (Apparently there's some sort of correlation between hemisphere dominance and the preferred direction, and between the ability to change direction and IQ score. I'm sceptical on both counts.)