what does buck say when Huck ask how the feud started between the shepherdson and the Grangerfords got started?

Twain says it best; straight from the text-

"Well," says Buck, "a feud is this way: A man has a quarrel with another man, and kills him; then that other man's brother kills HIM; then the other brothers, on both sides, goes for one another; then the COUSINS chip in -- and by and by everybody's killed off, and there ain't no more feud. But it's kind of slow, and takes a long time."

"Has this one been going on long, Buck?"

"Well, I should RECKON! It started thirty year ago, or som'ers along there. There was trouble 'bout something, and then a lawsuit to settle it; and the suit went agin one of the men, and so he up and shot the man that won the suit -- which he would naturally do, of course. Anybody would."

"What was the trouble about, Buck? -- land?"

"I reckon maybe -- I don't know."

"Well, who done the shooting? Was it a Grangerford or a Shepherdson?"

"Laws, how do I know? It was so long ago."

"Don't anybody know?"

"Oh, yes, pa knows, I reckon, and some of the other old people; but they don't know now what the row was about in the first place."

http://ask.fm/KyriakosKyriakou519/answer/69953613281

This makes lots of sense.