As u will no doubt have seen for yourselves on the odd occasional visit to Riften there will a fight going on between the Riften guards and the townsfolk versus a thief when I went there yesterday I seem two imperial guards fighting and eventually killing a Riften guard but the townsfolk didn't join in they just stood around shouting things lol

My last run through which included the last dlc also had two imperials fighting three riften guards, although only one guard from Riften actually fought and she beat the crap out of the imperials. Is this somerthing about riften that we don't know about - when you talk to the Jarl she seems to support the imperials more than the storm cloaks (or that's the impression I get).