What is happening to my plastered walls??

The plaster on some of my external walls has been bulging/popping in random areas over the years resuting in both the render and plaster coming away. The wall, render and plaster is bone dry. However at the bottom of the conical holes there is a dark crystalline substance. It looks like some kind of chemical reaction has taken place causing the render to bubble and displace the plaster. I have attached some photos of the holes.