Its just dissimilar metals and galvanic corrosion. The further each metal is away from each other on the periodic table the worse it gets.
Here is a simplified table.
http://irongrain.lowercaseproduction...-Corrosion.pdf
You can help prevent it by putting some Dielectric grease on the contact areas. Also on the plug wires contacts, the dizzy cap and each spark plug.
https://images-na.ssl-images-amazon...._AC_SX466_.jpg