This is only a guess that would require testing to confirm, but the natural gas pipe may be cathodically protected, either with buried anodes or by being connected to a rectifier system that impresses a DC voltage on the pipe. Usually gas pipes are coated with a paint that is supposed to insulate the cathodically protected pipe, but if this coating is damaged, the DC current from the cathodic protection could flow to the water pipes and cause the leaks you describe. As a precaution, I suggest you sleeve the new water pipes with a length of PVC conduit that extends well beyond the tunnel where the gas pipe is located to prevent this from happening again.
Jim Cook
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