How does Ulrich's attitude toward his rival change in The Interlopers?

Faced with possible death, Ulrich von Gradwitz decides to no longer hold onto his animus. Instead of this enmity toward Georg Znaeym, who is pinioned beside him by the fallen tree's branches, Ulrich offers friendship.

After the split second of being vis-à-vis as they both have rounded the huge beech tree, the men are held captive by the felled tree. At first, each man hurls invectives at each other, but as they await their own men to rescue them, this life-and-death situation puts much more valuable conditions into perspective. Ulrich von Gradwitz is the first to speak, offering his flask of wine, saying, "....Let us drink, even if tonight one of us dies." But, Georg Znaeym repels his offer,

"No, I can scarcely see anything...and in any case I don't drink wine with an enemy."

Hearing this, Ulrich ponders their situation for a while as "an idea was slowly forming and growing in his brain...." Despite Znaeym's hostility, he turns to Georg and tells him that he has changed his mind, and if his men arrive first, he will have Znaeym treated as his guest and freed first. Further, he tells his "neighbor," as he now calls Znaeym, that they have quarreled all their lives about "this stupid strip of forest" where trees cannot even withstand the wind. He concludes,

"Neighbor, if you will help me to bury the old quarrel, I--I will ask you to be my friend."

Silent for a time, Georg finally speaks in "jerks," remarking on how the region would be shocked if they rode together into the market square. He, then, displays a change of mind, and accepts Ulrich's offer of friendship: Asking for the wine flask, adding, "Ulrich von Gradwitz, I will be your friend."

In "The Interlopers", what triggers Gradwitz's change in behavior?

Ulrich von Gradwitz is upset because Georg Zaenym has no respect for the fact that Gradwitz has legal right to the narrow strip of land that is filled with wild game for hunting. Georg continues to hunt on Ulrich's personal property. This infuriates Ulrich von Gradwitz. He becomes obsessed with hunting Georg down to kill him.

On this particular night, Gradwitz gathers a group of his men in order to hunt down Georg:

On this fateful evening, Ulrich gathers a group of foresters to patrol the land in search of Georg. Separated from his men, he hopes to meet Georg alone and, when he steps around a tree trunk, he gets his wish. The two men face each other with rifles in hand, but neither can bring himself to shoot the other.

Gradwitz is tired of Georg trespassing on his land. He is determined to stop Georg from hunting on his land. Gradwitz is seriously contemplating killing Georg. However, once he finds himself face to face with Georg, Gradwitz hesitates about shooting Georg. Lightening strikes a tree and pins Georg and Gradwitz, ultimately saving Georg's life.

Ironically, Gradwitz and Georg have time to think about the foolishness of their feud. They decide to become friends, at Gradwitz' suggestion. Gradwitz realizes that he is just as helpless as Georg is. During this time, while both men are pinned, Gradwitz has a change of heart. He decides that the feud should end. While being pinned, Gradwitz felt hopeless. He sensed that the feud had gone on long enough. Georg agreed with Gradwitz. As they put their feud to an end, figures are coming toward them. Thinking this is the rescue team, Georg is relieved. Gradwitz can see more clearly from his angle. It is a pack of wolves coming toward them.

Gradwitz may have had a change of heart a little too late.

Why does Ulrich's attitude toward his rival change in "The Interlopers"?

After being pinned beneath the huge branches of the beech tree, Ulrich begins to reconsider his enmity toward Georg Znaeym as he is faced with the possibility of death.

On a bitter winter night with a cruel wind blowing, Ulrich von Gradwitz decides to hunt for his enemy, so he gathers his men and ventures out in order to search for the "prowling thieves" he suspects are hunting in the contested strip of forest over which the two families have feuded for generations. Ulrich breaks away from his men, hoping to happen upon his enemy with no one to witness him.

Ulrich comes around a huge beech tree and abruptly faces his enemy. In this moment of sudden recognition, Nature interferes, breaking branches with a violence that sends the branches crashing upon the two men, pinning them beneath the tree's weight.

Imprisoned and bloodied by the branches, the two foes can do nothing to each other. They initially curse and threaten each other with retaliation when their men catch up to them; however, after some time in which they have struggled uselessly, Ulrich is able to pull out his wine flask:

The wine was warming and reviving to the wounded man, and he looked across with something like a throb of pity to where his enemy lay, just keeping the groans of pain and weariness from crossing his lips. 

Apparently, when he finds himself in such a life-or-death situation pinioned with his enemy, Ulrich rethinks the importance of this hatred, and he decides he should live in peace rather than enmity. Feeling pity for Georg, Ulrich offers him some of his wine. Then, as he considers things further, he asks Georg to be his friend.

Realizing that he and Georg are facing a life-and-death situation, Ulrich weighs the importance of life against the significance of a feud, and he finds that their lives and welfare are more important than their families' grudge.

In "The Interlopers," what causes Ulrich von Gradwitz to change?

If we look at the story, we can see that it is the rather unfortunate situation that Ulrich von Gradwitz finds himself in that leads to his change of heart. The way in which a tree crashes down on both of them, trapping them and pinning them to the floor seems to change everything. For Ulrich von Gradwitz, his change comes about when he manages to free his arm and get to his wine flask so that he can drink some of it. Notice what he thinks as he has something to drink:

...the wine was warming and reviving to the wounded man, and he looked across with something like a throb of pity to where his enemy lay, just keeping the groans of pain and weariness from crossing his lips.

It is therefore when Ulrich is able to drink and to appreciate the benefits of a bit of wine that causes him to look at his enemy and to feel pity for him in his plight. Being trapped in a compromising position with his enemy is therefore the catalyst that causes Ulrich von Gradwitz to change his views regarding his adversary.

What is Ulrich's initial reaction to the new situation in "The Interlopers"?

Ulrich is initially thankful to be alive but also completely frustrated at being in this position. George laughs when Ulrich mumbles these contradictory feelings of thankfulness and exasperation. Ulrich is still in the mindset that George is his sworn enemy. He warns George that when his (Ulrich's) men come, he will be in even more trouble for poaching on Ulrich's land. George retorts in kind, saying that his men may arrive first and will find it easy to roll the tree over Ulrich. Ulrich responds with the same threat that his men will arrive first and kill George. Both men are still enemies, swearing death and damnation to each other. 

Their attitudes do not change until Ulrich offers George a drink of wine, still indicating that one of them will be killed. George refuses but Ulrich's gesture and predicament provoke a change in him. He swears not to have his men kill George. George responds that he will do the same and they determine to be friends, ending the feud, as long as no interlopers interfere.