Artistic decisions drive part of this; the photographic context may have determined other parts (specifically, it's possible the photographer wanted to be unobtrusive and unnoticed); physical compromises determine the rest:
Obviously the photograph was intended to be in grayscale, not color. Salient elements of grayscale quality are contrast, sharpness, and grain, all of which were influenced by the exposure settings and ambient light, as the subsequent points will describe.
f18 was needed to achieve a depth of field that put the entire face in focus. Because the depth of field is still fairly narrow, we infer that a telephoto lens was used--perhaps a fairly extreme one (c. 500 mm on a DSLR?).
Because there was no apparent action to freeze and because f18 on a standard-format camera would involve substantial diffraction blurring (which sets a limit on how sharp the picture could possibly be), there was likely no need to use a short shutter speed. However, a long focal length would require perhaps 1/500 to 1/800 second exposure to eliminate blur from camera vibration. That would have allowed an ISO of 1600 (or even smaller ISOs, by increasing the aperture a bit). So ISO 3200 may have been a choice, not a technical compromise. It created a certain amount of noise. This produces a slight grain that is characteristic of B&W film. However, this grain is extremely subtle--the sky is clear and grain is apparent only in the mid-grays of the background (a fence)? So another possibility is that the photographer was rigged to "snoop" at a distance, using a long telephoto lens, long depth of field, and very short shutter speed to allow quick snapshots of anything interesting that appeared quite some distance away without having to fiddle with exposure settings. "Stealth photography," if you will.
The photo was shot in bright sunlight either by necessity or to achieve the rugged, high-contrast effect.
Finally, the shutter speed was determined by the combination of light intensity, aperture, and ISO.