In photo taken with a prime lens, what is the cause of the "zoomed" bokeh appearance?

This image was taken with a Vivitar 24/2 (Kiron) prime lens. The lens is sharp. The focal plane is reasonably flat, and there is barrel distortion.

The bokeh balls have an oval shape in which the long axis appear to radiate from the center, as if they were created by "zooming". What is the name of this appearance? What aberration or distortion causes it?