Why Ulrich and Georg are enemies?

The two men are enemies because they are involved in an inherited dispute over the border-forest, and the game therein.

The neighbour feud had grown into a personal one since Ulrich had come to be head of his family; if there was a man in the world whom he detested and wished ill to it was Georg Znaeym, the inheritor of the quarrel and the tireless game-snatcher and raider of the disputed border-forest.