This is a good question. The short story offers two reason for the anger of Ulrich and Georg.
First, the story says that jealousy was one cause for the hatred and anger of Ulrich. Ulrich's family, wealthy land owners, won a case in which disputed lands were awarded to them. However, Georg's family never accepted it. They continued to use the land, even though it was poor in almost every way. This caused great jealousy for Ulrich and his family. Here is the quote:
The forest lands of Gradwitz were of wide extent and well stocked with game; the narrow strip of precipitous woodland that lay on its outskirt was not remarkable for the game it harboured or the shooting it afforded, but it was the most jealously guarded of all its owner's territorial possessions.
Second, the story says that the men also took things personally. In the end, this caused immense anger to the point of each man wishing great ill towards the other. Here is what the text says:
The feud might, perhaps, have died down or been compromised if the personal ill-will of the two men had not stood in the way; as boys they had thirsted for one another's blood, as men each prayed that misfortune might fall on the other, and this wind-scourged winter night Ulrich had banded together his foresters to watch the dark forest, not in quest of four-footed quarry, but to keep a look-out for the prowling thieves whom he suspected of being afoot from across the land boundary.
Even if the text does not say it, we can say that pride was probably the main cause of animosity. Hence, the land is not the issue at all; it is about the pride of each man's heart.