There was a bad bacth of stainless steel that SA had a while ago. I think it was 2 or 3 years ago. It happened to a number of people and the problem has since been fixed. All of the new stainless guns appear to be holding up well!
I guess that picture will never give up the ghost after all it has been around a long time ...... Best answer, it is a stress fracture and on a 1911 slide that would be the best/easiest place for it to brake off.... The reason this place would be more prone to breaking then any other place on the slide is because if you notice that is where the dust cover ends and if you notice a 1911 cycling thats where it gets hammered shot after shot.... And be aware this could happen to any 1911, on any note I would bet it sure would hurt if it popped loose on recoil...
I'm one of those engineer types. Of course, a Chemical Engineeri, not a materials, but I've done a fair share of failure analysis on bearing and shafts for pumps (plus a night or two at a holiday inn express) so I'll register a theory.
Probably bad steel, or bad heat treat.
Alternate: for one of any number of reasons the slide was coming back and slamming into the spring guide/recoil surface. This could cause a stress point right at the point where the top of the slide and the dust cover portion of the slide meet. Do this enough and get stress cracks. Do it a few more times and get a real crack. A few more and get this picture.
Its hard to tell for sure from the picture, but the dust cover part of the slide looks pretty clean, as to indicate no battering. Without holding the piece in hand its hard to say for sure though. Flanging of the end surface would not be very evident from the angle of this picture.
Again, just a theory as I'm no expert in this field.
Imagine having to call the range cold to go get your slide!