See the bright spot on the top of this solder joint? That's a new concavity in the solder. I.e. the spark melted and splashed the solder. Now, an interesting note about this: you don't get that splash on the side where the wire first came into contact with the board, only the second one to touch. This happens repeatably. This is notable because it means that the air between the wire and the second contact point is ionizing. I.e. this literally creates a miniature lightning bolt.