My guess (and is it only a guess because I have not seen the slide first hand) is that the frame rail pushing down on the lip in the slide is that caused that failure. It looks like the the edge of the slide is pushed down toward the frame. It makes sense that the lip would fail there because it is not continuous because of the cutout for the slide release. It would be interesting to look at the crack under a microscope to see if it failed due to fatigue or if it was a catastrophic failure. Although because of the thin cross section it may be hard to tell.