The article hasn't described the root cause but Musk had hinted at it in one of the interviews. I think what happened was that when gigafactory started, they ha

The article hasn't described the root cause but Musk had hinted at it in one of the interviews. I think what happened was that when gigafactory started, they had this huge vision of complete end-to-end automated assembly. So there were robots designed for each portions of assembly, including some robots which were supposed to operate in inperfect situations through vision data, for example, locating a dangling hose, grasping it and connecting it within a car. The big learning was that robotic functions that operated in uncertain environment, relied on vision and tried to perform manipulation tasks failed big time to do the job consistently. Musk mentioned that human would pick up a gangling hose and connect to its expected position effortlessly in a second while a robot will fumble around and dig in to car even damaging it. In past few months, I think they have put a lot many more humans at work turning in to more traditional way of assembly, although more automated than other car manufacturers.