It's an example of sampling error. You'll see it typically in movies where you have a sampling rate of 24 samples per second. The rotational speed at which the wheel appears to be going backwards is going to depend on that sampling rate and on the number of things on the wheel (typically spokes on the wheel of a wagon or stage coach) that give you the detail to see the apparent rotation. If the wheel has six spokes and goes around almost one time or almost 5/6th or almost 1-1/6th of a full rotation or variations on this (e.g., almost 3-1/2 times) it will appear that it has gone backwards just that little bit less than whatever multiple of 1/6th of a rotation it did in reality. Watch the wheels as they are picking up speed. The sampling rate in TV is thirty samples per second.

Confused yet?