Why does Winn-Dixie have pathological fear?

Winn-Dixie has a pathlogical fear of thunderstorms. His fear is inexplicable.

The preacher lay there on the ground, rubbing his nose. Finally, he sat up. He said, “Opal, I believe WinnDixie has a pathological fear of thunderstorms.” And just when he finished his sentence, here came Winn-Dixie again, running to save his life. I got the preacher up off the floor and out of the way just in time.

There didn’t seem to be a thing we could do for Winn-Dixie to make him feel better, so we just sat there and watched him run back and forth, all terrorized and panting. And every time there was another crack of thunder, Winn-Dixie acted all over again like it was surely the end of the world.