Why are Ulrich and Georg feuding ?

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.