Put a semi-literate or barely educated young fellow of sixteen to twenty out in the middle of nowhere, which is where most of those cattle drives went, bore him to death one minute and scare hell out of him the next. Let him witness the maiming or violent death of a friend or two. Keep him in the saddle, choking on trail dust, for 12 or 14 hours or more, day or night, rain or shine. Feed him nothing but rancid bacon, beans, weevily buiscuits and coffee for about three or four months. Then, take this testosterone-charged boy, give him a few bucks and turn him loose in a town full of card sharks and "soiled doves." Talk about ample source material...

He probably had little to no musical ability or training, but a rough sort of cowboy poetry was a common way of sharing these experiences in the west, so maybe he wrote or memorized a few verses. Since he couldn't write music, he probably borrowed a familiar air, related or not. Shared, first, around campfires, the songs take on a life of their own, often modified by succeeding singers.

That's my version, and I'm stickin' to it.