This is besause video is not one continuous moving picture but a series of stills, an effect called "persistance of vision" takes place in our minds to make it look seemless. If the number of frames per second matches the frequency of a certain movement, the object appears still, faster of slower produces a strobe effect which can make an object appear to be moving in reverse of its actual direction. Doug Babcock surely knows more than me about this but I think my description is fairly accurate.