Was the attacks against the Klingon colonies random acts of cruelty by the Romulans or in response to unchecked Klingon expansion.
Might have been something much more specific. Perhaps Khitomer
a) was a sore point for a certain Romulan family that had invested a lot in the local agriculture once
b) featured a rare, rich deposit of tri-mysterium
c) was suspected of being center of bioweapon development
d) was home to a Klingon individual who stood in the way of closer Klingon/Romulan cooperation
e) harbored a Romulan defector