You were in aperture priority mode, so the shutter speed would have been set automatically by the metering (pattern mode, according to exif).

I think the real question is why did the camera then reduce the shutter speed on the flash versions, and/or why didn't it change shutter speed when you changed the ASA to 400 (assuming light conditions remained reasonably constant). Theres nothing in the foreground that would have reflected the flash and I guess you had focus at infinity (exif says subject distance unknown for all 3 images, and no reflected light from flash detected).

I'm sure one of the Nikon guru's can shed some light on things ;-)