Also, Romulans can carry a grudge longer than humans, as they supposedly have longer lifespans (and perhaps also longer generations, although that is neither here nor there). The Commander and Centurion of "Balance of Terror" spoke of serving together in numerous campaigns; since they would be imprisoned inside the Neutral Zone that supposedly separated Romulus and Remus from the rest of the universe, they could only be referring to infighting - or then to the old war with Earth... Perhaps the Commander had once slain a Stiles or two?
The hundred years of "silence" might in practice consist of a hundred years of subspace dialogue, in which the Romulans sounded like a broken North Korean propaganda record. That'd keep certain humans permanently pissed off, too!
You're assuming that simply because the Romulans had no contact with the Federation, that they had no contact with other neighboring species.
The Romulans were pretty supposedly isolated from the Federation again in much of the 24th Century, but still had frequent conflicts with the Klingons (Naredra III, Khitomer Massacre, the attack in which Martok got a battlefield promotion)