The answer is already given by @DHFCO mentioning zinc as the perpetrator.
Low melting solder is more a trouble maker then hard solder due to the amount of zinc in the solder as DHFCO already mentioned.
This phenomenon is called "zinkfras" in the German language caused by too high solder temperature.
Zinc starts to boil creating the little bubbles.
Remedy? Remove the bad part and use hard solder.
Soldering is a procedure of temperature control and recognition when to step in with the solder preventing the boiling of your solder or better the zinc used in your solder. Working with solder is knowing what you're doing and how the metals behave in a alloy. It's not rocket science but common sense and practice.