The feud is caused by an old land dispute between the Gradwitz and Znaeym families.
As the other educator pointed out, the feud is inherited by the succeeding generations of both families. The land in dispute is only a "narrow strip of precipitous woodland" on the outskirts of the Gradwitz property. It is not particularly "remarkable for the game it harbored or the shooting it afforded." However, the feud continues because of entrenched feelings of ill will between both families.
While Ulrich believes that the disputed land belongs to his family, Georg is of the opinion that his family's claims have been unjustly ignored. In Ulrich's eyes, the court rightly sided with his family against the machinations of "petty landowners." However, Georg rejects Ulrich's perspective, and this is why he continues to encroach on Gradwitz land. Each makes it his life duty to kill the other. The feud is sustained by strong emotions and petty jealousies.
Every day, Ulrich and his team of men search the woods for "prowling thieves." Despite his conscientious reconnaissance work, Ulrich only comes upon Georg once: during a tempestuous, stormy night. The chance meeting is almost surreal; neither man can believe the object of his consuming hatred finally stands before him. Interestingly, neither man immediately acts upon his murderous intentions. It is impossible to ignore the "code of a restraining civilization" even in the moment of great provocation.
This hesitation seals the fate of both men. An enormous tree crashes, pinning Ulrich and Georg under its formidable branches. In the end, both men discover that an inherited feud is meaningless in the face of a possibly violent death.