Why does Nat think there are witches around?

Nat gets a glimpse of a few of the hounds under Jim's bed, and then the room fills with more as they come in through the lean to...... Nat is completely spooked by the dogs. Also, it doesn't help that Tom plays along.....

And whilst we was a-standing there in the dimmish light, here comes a couple of the hounds bulging in from under Jim’s bed; and they kept on piling in till there was eleven of them, and there warn’t hardly room in there to get your breath. By jings, we forgot to fasten that lean-to door! The nigger Nat he only just hollered “Witches” once, and keeled over on to the floor amongst the dogs, and begun to groan like he was dying. Tom jerked the door open and flung out a slab of Jim’s meat, and the dogs went for it, and in two seconds he was out himself and back again and shut the door, and I knowed he’d fixed the other door too. Then he went to work on the nigger, coaxing him and petting him, and asking him if he’d been imagining he saw something again. He raised up, and blinked his eyes around, and says:

“Mars Sid, you’ll say I’s a fool, but if I didn’t b’lieve I see most a million dogs, er devils, er some’n, I wisht I may die right heah in dese tracks. I did, mos’ sholy. Mars Sid, I felt um—I felt um, sah; dey was all over me. Dad fetch it, I jis’ wisht I could git my han’s on one er dem witches jis’ wunst—on’y jis’ wunst—it’s all I’d ast. But mos’ly I wisht dey’d lemme ‘lone, I does.”

Tom says:

“Well, I tell you what I think. What makes them come here just at this runaway nigger’s breakfast-time? It’s because they’re hungry; that’s the reason. You make them a witch pie; that’s the thing for you to do.”