I asked the plumber who repaired the leak today, that very question. He explained that the leak was caused by "electrolysis." He also said that there is way of knowing where, when or if, it will occur again. He replaced most of the copper with plastic line that he has a lot of faith in. In fact, he pushed the new plastic through the inside of the old copper pipe. He then used a fitting to connect it with the remaining about 12 feet of working copper pipe. One problem is that remaining copper pipe runs under my blacktop driveway, where it connects to an old 2 inch plastic pipe. My thinking is that I'll keep paying for the waterline insurance and it fails again it's their problem to locate and fix the leak. I was told that they have specialty tools that will let them run a replacement line under my blacktop driveway. He was talking about needing to dig up a decorative tree to run a line under my blacktop driveway. I was just happy to have the water running again with just a little damage to sidewalk and not losing any trees. If the copper lines fails again and if I'm still alive, I reconsider the issue.