I'm pretty confused myself. How many chickens do you have, by ages and by sex? The better you can explain the numbers, sex, and ages the more likely it is to help me understand. How are they housed, inside and outside? Where does this happen? How much room do they have, in feet or meters or are they free ranging with no fences? Have you added (integrated) new chickens recently or maybe removed some to change the pecking order? This sound like a new behavior. If so, what has changed that might have caused it.
I think this means you have a bachelor flock (of unknown age) penned separately but on two different occasions a girl of unknown age got into their enclosure and was abused, possibly injured. When you put those girls back with the main flock the flock rooster or roosters mounted her. I don't know what you mean by "violently mounter to death", could you explain what you saw?
Could you explain what this remembering looked like? How did you reintroduce her and what happened?