We may never know exactly why dogs sometimes act this way. All we can do is make some assumptions. Things that come to mind are that in many households, once dogs reach social maturity (anywhere between 12 to 36 months of age) there is a tendency for relationships to shift and for aggressive behavior to raise their ugly head. Other things that come to mind are that maybe there may be something medical going on that may cause increased hunger in your male or perhaps he is not feeling too well which can lower the aggression threshold. Maybe one day when you fed the treats, the dogs were too close to each other and your male felt threatened by your female.
While dogs may get along fine, not many dogs will accept another dog too near to food and valuable treats. Distance is important to ensure everybody is comfortable and relaxed. I suggest giving them more distance when you feed treats so that they don't have to be worried. Even better, give the treats in separate rooms or in crates. Careful, as these episodes tend to repeat and may sometimes happen over toys or other valuables.