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I would say that if there was a female in heat and he is not neutered then that would make a fight happen.
Dogs can smell a female in heat for almost two miles and they will become aggressive.
At the same time you are right in assuming the possibility of him seeing the other dog in a weakened position would also cause a dog to try an take over.
I would suggest that you keep an eye on them over the next few days to see how it goes as a female will be in heat for at least 4-9 days. If things calm down then you will know that was the problem.
I see you are offline right now so I will sign off and wait until you have the time to come back to the conversation.