They were probably soldered with too much, or too strong, acid flux. Some fluxes are too aggressive for copper piping, since they don't stop etching the metal. I've seen that happen with electronic items, too, when someone uses acid flux instead of rosin flux.
My parents' house had copper waste plumbing, built in 1958, never had a problem. Sold the place 5 years ago.
I had a supply pipe leak when it developed a pinhole in it, and it sprayed water around for a while. When I examined the pipe, there was a nodule on the inside, blocking a lot of the flow, and the pinhole. Turns out the phone company used that pipe for a ground near that spot, and the electrolysis ate the pipe. It took about 40 years to do it, though.
If you repair the pipes yourself, here's a little tip: there always seems to be some water dripping down the pipe, no matter how well you drain them. Take some white bread, ball it up, and stuff it in the pipe. It will stop the dripping long enough to solder the joints, and when you turn the water on, the bread will just flush away.