If the boys can still smell "woman" on the others, they are more likely to fight. I also have never had any of my boys fight. I have 2 males actively breeding at any given time, and 3 others waiting on the bench. When they get reintroduced, it's in a clean tub that none of them have been in, and I have yet to have any problems.

Did you hear them fighting or watch it happening? And how much blood was there? Males and females have to reestablish the pecking order, as all rats have a social status within the colony.