I'm betting there was more than one reason. Stockholm syndrome being one and uncertainty about the grounds for the Nord invasion seems reasonable. What did the Bretons know about them? For all they did know, the Nords had the potential to become another enslaver and in that case it could have been a matter of the better known evil. Also, perhaps fear of the Elves who held their lives in their hands. If the Elven masters told them to do something, did they have a choice other than death? Or worse? And of course, what do we know about the Breton slave culture and the way it meshed with the custom of the Elves at the time? Maybe those doing the fighting against the Nords did so because they wanted to. Perhaps they weren't all actually slaves or had been brainwashed into thinking it was their duty.