Horses attacking a foal

This question is so upsetting that it was hard for me to write. I have looked on line and could not find the answer. I asked the Vet why horses do this and he said "because their horses". I am loosing sleep and still traumatized over what happened at the stable where I board my gelding.

A  mare gave birth to a baby filly and somehow was she able to slip through the fence in her first hours of life and was found later that morning bitten and kicked. She was rushed to the Vet but later died of a fractured skull. I know it was human error that this was allowed to happen but can you help me understand why the other horses did it? I know it is not my geldings fault if he was involved but am still feeling horrible that it may have been my horse that did it.

In the pasture was an alpha mare, her yearling gelding, and my 6 year old gelding. When the filly was found my gelding was keeping the other horses away from her. 

I'm sorry to tell you this sad story but appreciate any bit of information to help me deal with this tragedy.  

Thank you, Madelyn