Zinc is the metal to put the blame on.
People used to call it “zinc gobble” or “zinkfrass” (german) Zinc
starts to oxidize -at high temps- leaving pits behind in a joint. As
others already mentioned, this is caused by overheating and/or
extended soldering procedure.
For the ones who make their own solder, zinc gobble may be caused by
incorrectly melting of your metal ingredients.
Roll the material to thin pieces and cut them in small pieces, heat
your (boraxed) crucible and then insert the solder.
Cover it with a mix of charcoal and salt. Melt your solder quickly
(!) Poor an roll to the wanted thickness. This results in a fine
solder of good quality.
Adding a few flakes cadmium can help aswell (which I don’t recommend
for beginners due to the toxicity of cadmium). Cadmium prefends the
oxydation of zinc according a recipe I have from older books. Only
work in a very good ventilated room and take precautions needed for
Have fun and enjoy