This was inspired by a boxed set of The Wire, which Bugg was watching. He told NME: "I could see parallels between Baltimore and where I grew up, plus it introduced me to a load of drug references, which sparked the idea for 'Kingpin.'"
The kingpin is the pin at the center of the triangle of pins in skittles, bowling and other similar games. Its name arises from the fact that if struck successfully, the kingpin knocks down the others. Figuratively, the word is applied to the principal person in a group or company, including in this instance the person in charge of a criminal organization.
Who writes a song about a name they found in a phone book? That's just one of the everyday things these guys find to sing about. Anything in their field of vision or general scope of knowledge is fair game. If you cross paths with them, so are you.