I had two horses for years. I broke all the rules.. one was a gelding and the other a mare. When I rode one out, the other would run and scream. I ran a hot wire on the fence so that the one left would stay put (not that the one left ever got out.. just a precaution).
The horse I was riding was NEVER a problem, even the green ones. The one left was the one carrying on.
Advice is to not pasture mares and geldings together.. or have enough riding and training skill that whatever horse you are on is thinking about you and not about the one screaming. The mare is not "doing this" to other horses. She is just being herself.
The horse you are on needs to be focused on you.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer