Obviously the leaks here are a lot more serious. My experience of significant leaks in spacecraft propulsion would lead me to believe that there are welding issues. It can be very difficult to weld some of the materials used in these system, adding to the fact that they are all bound to be very thin walled. Cracks in welds can be hard to find, although I would expect every weld in a system like this to be X-ray inspected, and maybe also penetrant tested.
On the projects I have seen with issues like this, the main culprit has been steps in a procedure being skipped. Inspections signed off despite not being performed. I would highly doubt that this is the case here though, even with Boeings recent track record. Human spaceflight is treated different.