The plumber decided it was a good idea to solder the pipes to the pipe supports. Each one had corrosion. I'm thinking that the solder increases the amount of galvanic corrosion between the dissimilar metals by providing a better electrical connection.
"Supply pipes in the main crawlspace had visible corrosion where they are soldered to supporting hardware. Corrosion appears to be the result of dissimilar metals electrically connected by solder resulting in galvanic corrosion. This condition may result in eventual leaking and these connections should be corrected or monitored for future leakage."