It can be anything that is causing the corrision and pinhole leaking. There really is no proven reason as to why it happens. Some experts say it is caused by mineral content in the water, to as much as grounding problems. The only thing you can really do is to maybe look at areas that that are showing signs of possible corrosion and keep an eye on them.
Also, if the ground is going to the cold water pipe, but you have PVC from the house to the main, of course there is no grounding between the house piping and the meter, which means any stray current has no place to go, which is also something that experts say can cause corrision.