The reason that these two men hate each other goes back pretty far in time. It goes back to the time when the grandfathers of the two men were in control of the lands that Ulrich and Georg now control. It seems that von Gradwitz's grandfather managed to use the courts to get this land away from Znaeym's grandfather.
Here is a quote that shows this:
A famous law suit, in the days of his grandfather, had wrested it from the illegal possession of a neighbouring family of petty landowners; the dispossessed party had never acquiesced in the judgment of the Courts, and a long series of poaching affrays and similar scandals had embittered the relationships between the families for three generations.
Because of this ancient feud, the two men hate each other. This is why they are out patrolling on the night that the storm hits. Therefore, it is that old lawsuit that is going to (it seems) get them both killed.