There is no other place for a crack to start other than the weld. Unless the tube suffered some insult that is not apparent in the picture we have. But it clearly started out of the picture, and probably on the side of the bb just at the edge of the weld. It then traveled perpendicularly to the load path. That weld looks shady to me, like there is poor penetration at the edge. But everyone on the internet is a weld critic.