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.