I've just come across two men, one early nineteenth century, one later nineteenth century, whose first name was 'King'. With the first one (the excellently named King Sparks), I assumed his mother's surname was 'King'. Now I've come across the second one (King Huggeston), I'm starting to wonder if this was actually a name? Or perhaps just a coincidence, and he too had a relative whose surname was King.
The normal naming convention was to give the first born son his mother's maiden name as a middle name rather than a first name. That has certainly been borne out by my research anyway.
It could just be a favoured family name. Sometimes children were named after a wealthy relative in the hope that they would --leave them a wodge of cash-- look kindly on them.
Some families just have names they like to hand down from generation to generation. I worked with a woman whose DH's family insisted that all the first born boys were called George. She called her first born DS Oliver