Just because kids, and animals are not stupid, and may even know better, doesn't mean they won't get a brain fart, and do something stupid.
You mentioned that you had been giving a verbal command, on a daily basis, to the roosters, so they wouldn't fight. Maybe you forgot to say it that day.
I had a Poodle named Pepper, for 14 years. Pepper was obedience trained, and could do several tricks. He knew what "heel" meant, and never needed a leash. We moved several times, because of dad's job, so even if the yard was not fenced, we would walk Pepper in his yard, around the perimeter a couple times, telling him "stay in your yard". After that, when we'd let him out, all we had to say was "stay in your yard" when we'd let him out, and he would not leave his own yard.
When we moved back to Florida, the house my parents bought was next to a big cow pasture. The fence wasn't up yet. We walked him around the perimeter a couple times, told him to stay in his yard, and he did. One day, whoever let him out, forgot to tell him "stay in your yard". Pepper didn't forget. He bolted to the cow pasture, found the stinkiest cow pie he could find, and rolled all over in it. He was covered in stinky cow pie. When his name was called, he came running back, and couldn't be let into the house without being bathed first. What a mess.
I don't care if you said "stay in your yard" for a couple years, and he stayed in his yard just fine. IF you forgot to say it, it was guaranteed that he had not forgotten, and would come back covered in stinky cow pie. It happened less than 5 times throughout his lifetime, but he taught us that you didn't want to be the one to let him out, and forget to say "stay in your yard", or you were going to be the one dealing with the mess, and giving him a bath.
Lesson: Don't forget to tell the roosters, every single day, not to fight. LOL