Coriolis effect. Same reason drains swirl spontaneously, and all in the same direction in each respective hemisphere. Also Earth is an oblate spheroid so the effect is even more pronounced as you approach the equator.

I don't know if it's been done but my guess is the same dude in the same experiment in the northern and then southern hemisphere would circle in opposite directions.

That's my theory anyway.

No, I took a class on this during wilderness survival training when I was with the air cadets. Humans all have a dominant leg, usually the same one as your dominant hand. When wandering in the woods (or even trying to hold a compass course) you will subconsciously circle in one direction or the other, with your dominant leg turning you slightly (to the left if your dominant leg is right).

You need to be aware of this and consciously compensate, by walking deliberately around the right side of trees more often than left.