In "The Interlopers", why is Ulrich so angry at Georg for trespassing on a small worthless piece of land?

Ulrich and Georg come from families that have been at odds with each other for many years; both have shamelessly cultivated to the present day, the 'personal ill-will' characteristic of the feud of past generations.

... as boys they had thirsted for one another's blood, as men each prayed that misfortune might fall on the other...

So, why does Ulrich get so angry at Georg for trespassing on a seemingly worthless piece of land? Ultimately, this anger seems to originate from all the emotion and prejudice engendered by a bitter court battle between both families in regard to ownership of the contested land.

...A famous law suit, in the days of his grandfather, had wrested it from the illegal possession of a neighboring family of petty landowners; the dispossessed party had never acquiesced in the judgment of the Courts...

Both men appear to have inherited the hatred of past generations without questioning the need to continue their current enmity. When Ulrich and Georg come face to face in the forest, both hesitate in the face of an opportunity to 'give full play to the passions of a lifetime.' 

But a man who has been brought up under the code of a restraining civilization cannot easily nerve himself to shoot down his neighbor in cold blood and without word spoken, except for an offense against his hearth and honor.

The men have dutifully inherited the bitterness of past generations; neither can point to any slight or insult in their present circumstances that would warrant such ill-conceived hatred. Their enmity is sustained by strong emotion and a misguided sense of loyalty to ancestral honor. In the face of suffering and ultimate death (both are pinned by a fallen beech tree), Ulrich finds that he cannot sustain his own hatred; the will to survive is greater. Ultimately, both men make their peace, but ironically, a possibly bitter end awaits them as they glimpse what they think are figures running to their assistance.

"Are they your men?" asked Georg. "Are they your men?" he repeated impatiently as Ulrich did not answer.

     "No," said Ulrich with a laugh, the idiotic chattering laugh of a man unstrung with hideous fear.

     "Who are they?" asked Georg quickly, straining his eyes to see what the other would gladly not have seen.

     "Wolves."