They didnt wet the pads and used the wrong type of solder or soemthing like this. Meaning the bond for the solder is barely holding on. This is why bumps have the potential to break these solder connections on most if not all of these that have not been reballed. Leaded solder is more ideal but even leadfree would yeild better results than the factories mess up.
Ken told me that sega put out a memo when they just came out saying they are aware all the boards were defective and to RMA them until extra spare boards ran out of stock and sega wasnt planning to rerun a new production of boards. AKA they immediately gave up on the platform after the factory mistake or they only built to order with the games they sold and expected to sell and then gave up after finding out.