it would most likely start in the heat affected zone near the weld, unless there is something at the weld that would cause a stress riser. Fatigue cracks in ductile metals are usually pretty straight in cases like this, that doesn't surprise me at all. i think most cracks you see on Ti bikes are on round tubes, they are going to be different.

Not sure a weld repair is going to last too long, but I suppose it's worth a try.