It's rare for rear wheel obstructions to cause a surprise fall, though it may happen if the wheel suddenly locks up at decent speed, such as when a stick is jammed in the spokes and carried up to the stays.

However crashes due to steering issues are very common. Sticks, large pods like chestnuts, and round stones can cause a crash by rolling under the tire and displacing the front wheel sideways a few inches. Immediate corrective steering is needed, and if you're not alert or have a loose grip on the bars this can bring you down quickly and unexpectedly.

Crashes due to loss of steering control are far more common than those caused by something in the rear, but both can happen. These are the risks of riding on something which requires keeping the center of gravity directly over a narrow track.