You need to punish her before she kicks. In my
book, the thought is as bad as the action. If she pins her ears, get after her.
And one swat on the butt for starting to back towards another to kick is not
enough. She would have got religion right then and there. That does not mean
beating the crap out of her. Swat her forward, then put her into some kind of
work immediately. Circles, forward, what ever so she know that was NOT
acceptable behavior. If she is aggressive towards other horses on the
trail, and is getting away with it, then you have not establishing yourself as
being in charge.
Don't get you, her, or someone else or their horse
kicked or hurt....... nip it in the bud now..