A belated attempt by the independent company SIGMA to compete with the major camera manufacturers. As you may know, the first autofocus interchangeable-lens 35mm SLR system was introduced by Minolta in 1985, followed by Nikon in 1986, Canon, Pentax and Yashica in 1987, while SIGMA entered this market only in 1993.
Like Minolta and Canon, SIGMA had to develop a new electronic autofocus bayonet mount for their cameras from scratch. It is curious that it was originally dual: the outer mount to prevent vignetting with large aperture lenses and telephotos, and the inner one for all other lenses. However, not a single lens was produced for the outer mount, and with the advent of the SD-14 it was abandoned.
SIGMA did not produce SA-mount lenses exclusively for their SA series SLRs: these were always models intended for cameras from major manufacturers PLUS cameras of the Sigma SA system. But the model range was very wide: from 8mm fisheye to 1000mm super telephoto lenses, all with built-in AF and aperture control motors.
Development of new cameras and lenses with the Sigma SA mount was discontinued in September 2018.