With the reduction of light available (since the aperture size is reduced) the auto focus sensor that provides the focus indication, probably does not have sufficient light in which to fully operate. If I remember correctly, it operates off of contrast, thus with the reduced amount of light, there is an insufficient amount of contrast by which to make a determination.
In comparison to a more modern auto-focus lens (notice I did not say better), the camera controls the aperture, so it keeps it open during the auto-focus to help with the contrast detection. Sensing a lack of perceived contrast, the lens will go into focus hunting.