It is definitely strange to keep having that happen. I would pull off the rim strip and and give the inside of the rim a look over. There may be a burr in the metal around a spoke nipple hole or other place from manufacturing. I have also seen the slots on the backside of nipples damaged so that they have sharp edges from the tool slipping during assembly. I would take a small file and carefully cleaned up any sharp imperfections that could penetrate the rim strip and tube.