I think it's probably just whatever part of your brain you were using when the image popped up. A simple explanation (probably wrong, even if I'm convincing!) would be that the thoughts "rotating clockwise" and "rotating counter-clockwise" activate different halves (maybe with some overlap). Since both directions lead to the same observations, the prior is 50/50, but if you're using your left brain when you look at the picture (or if your left brain is more developed) it has better odds to get the activation. If you lose focus, the other half might get active enough to "steal" the activation and reverse the rotation. The only meaningful rotation would be the initial one, before your brain has time to think about it, introspect, etc. I guess it would average over a large amount of tries, but I'm not going to do this every hour of the day.
I can see it go clockwise or counter-clockwise. It starts in a seemingly random direction and changes sometimes, mostly when I lose focus or look away (which I find logical: once the brain loses focus, the direction of rotation becomes "up for grabs" again).
Edit: lol I just trained myself to be able to change the spin at will. First I have to look at the edge of the page so I see the figure on the side, then I can make her spin however I like, then I focus back on her and the spin sticks. Can't do it while focusing on her (yet).