Odd pinhole leak in copper pipe. Selective electrolytic corrosion?

When I returned to the Isle of Man from Glasgow I could hear a very quiet hissing noise, and a thorough investigation of the elusive noise revealed that there was a tiny pinhole leak in a copper water pipe. The thumbnail picture shows the leak with the water off and the residual water in the system draining out at low pressure.
I removed and replaced a short section of the damaged pipe and checked the wood for signs of long term water damage. It seems OK, but once dry I'll treat it with Cuprinol preservative as a precaution.
The interesting thing is that the pipe has a very tiny pinhole, and as you'll see in the video it has corroded from the inside in a very small point. There are also other similar corrosion points in the same section.
I wonder what causes it. Could it be contamination with steel particles or another metal that has caused local electrolytic corrosion over time?

I'm hoping that it's not going to cause more issues, because there's not a lot of space under the floor. To replace the pipe will involve lifting a lot of flooring and awkward pipe placement.

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