Looking carefully at the break, I suspect that a number of factors contributed to the bridge tearing off: thin top, scoring of the top (particularly at the front of the bridge), minimal lattice support in the central area of the bridge (which has the greatest pull from the strings) and inadequate gluing to the lattice braces. The fracture point is at the front of the bridge and is totally clean, which suggests that the scoring was much too deep in that area. Once that let go, the rest of the bridge simply tore away raggedly. Under string tension the bridge was pulled up slightly at the back and the weakened area from the scoring at the front acted like a hinge and...whap. If that "hinge" had not been present, one might see a slight hump behind the bridge and a slight hollow in front of the bridge under string tension. I would agree with James...incompetence.
1971 Yamaha GC-10 (Hideyuki Ezaki)
2017 Yamaha GC82S (Akio Naniki/Naohiro Kawashima)