I ran into the same problem when working in a substation over the summer. I think it was caused from my sweat dripping into the shot while I was loading it. I worked with another apprentice that had the same thing happening to him. Nothing quite like having molten lava exploding out of the mold. We started holding down the lid with a long screwdriver, nobody was burned, and it made every weld just that much more exciting. Good times with ground grid……