I guess it is a combination of things.
Looking at the crack, location and growth pattern it has to be a fatigue crack which started at the top side of the tube and not a crack due to a single moment of overstressing. I would therefor consider it unlikely that is was damage from jacking although this could have initiated the crack. I would rule out road debris as a cause for this.
For a fatigue crack to grow which can be at a very slow pace, there has to be a cyclic (tensional) force. I guess it is the horizontal component of the shock force exerted on the flange during the movement of the suspension. This force provides a bending moment (Force x height mounting flange hole) and thus tension almost exactly where the crack started, which is on the top side of the tube.
I would also assume that the weld of the flange might have decreased the skin thickness there locally , thus increasing the tension in the material.
It could there for very well be that other A frames, where the weld has been made slightly different, will not exhibit this cracking.
And lastly, the location of the shock mounting on the lower A -arm is not ideal as it as it is not in line with the wheel axle thus introduces a torsional force on the A frame as well.
Maybe others have some ideas as well?