My guess is that he was looking for cheap victories to shore up his military credentials and get the army off his back. Monomachos in particular was under serious threat of being deposed by various generals e.g. Georgios Maniakes who were generally disgusted with his lack of army chops. Maniakes, incidentally, would've been an interesting emperor (I'm sure there's speculation about it somewhere in this board's archives), like a slightly-more-insane-but-equally-dangerous-to-everybody-around-him Ioannes Tzimiskes.
Somehow, "cheap victories" and "battles in the Caucasus" sound like contradictory goals.
I think there is a thread mentioning it, but it didn't go very far. Too bad, George I the Terrible would be...interesting. Not good, necessarily, but interesting.
Marwanid amirs ruled the Diyar Bakr and some territory south of Lake Van. Mosul was the Uqalid amirate, which generally fought against the Byzantines and their Marwanid allies. It used to be ruled by the Buwayhids during the peak of their rivalry with the Byzantines over the Anzitene in the 970s and 980s, but in 996 the Buwayhid governor of Mosul was forced out as part of the ongoing, almost perpetual state of collapse the Buwayhid state was in.
Gotcha.
If Monomachos really had "army chops" as you put it, this sounds like it would be the place to beat up and absorb - add a nice solid chunk of northern Mesopotamia to the empire.