It could be from the black threaded top cap on the shock coming into contact with the frame when drooping out, I'm pretty sure mine have some rub marks there, also 2.5" Icon shocks.
This would bind the threaded stud against the frame and potentially snap it.
I remember this happening to someone else on the forum and that was their conclusion. I find it a little hard to believe that the axle 'dropout' force alone would snap the threads as the upper mount bushings flex a little during something like this and take some of the initial force. Especially if you haven't been jumping your truck, but I could be wrong.