I am concerned about your mare’s health, Christine. I can suggest a couple of things: Is it possible to erect a visual barrier between her and the horses next to her? If so, that will likely stop the behavior.
Mares will often become more aggressive at certain points in their cycles. I suggest that you contact your vet to discuss the possibility of hormone therapy to help decrease the aggression. Hormone therapy should be undertaken carefully and thoughtfullyand only if you can keep a log of her behavior to note if it improves.