Hmph. I've had two such crashes. (The other two were with cars, outside the scope of your poll.)

In one, recently, my tire got caught in a trolley track and I went over. Could be lack of skill, I'll call it loss of traction. After all, whatever the stupid reason, eventually I was falling over sideways -- no traction there!

In the other, long ago, I got distracted momentarily and hit a big hole and went over the bars. Lack of skill? Looking at girls? I don't even remember. I remember that I had no traction as I flew through the air, however.

So I voted "loss of traction."

RichC