Now I'm not trained in failure analysis, nor am I an engineer, just a mug mechanic but it looks like the crack has propagated from around the 2:00 o'clock position from a stress raiser of some sort.
Now it could start as Rudi suggested, or the lead or start of the thread (threads are notorious for this, particularly cut threads if the tool is a little blunt) or just some sort of notch, and grown from there.
As threads get courser, we put a Larger or MORE blunt insert to Prevent the thread from weakening. The root has a larger diameter and the radius makes it stronger.
Just sayin. ;-))