Fence fighting is common, and very natural for territorial guarding by most breeds. The thing I have noticed is Roman will fence fight with only those dogs that are "out of control" from owner. Out of control simply means anyone that has a dog off leash, walking in front of owner, or owner lets walk up to fence to say "hi". Any dog that walks next to or behind owner is considered in control. The subtle body language cues used by dogs that most people are oblivious to can give you a good insight into their behavior.
Prey drive can also be a great tool in training.