Agree with others. It sounds pretty normal, but IF it's truly random - not that you're missing subtle signs or such, agree also she could be cranky due to intermittent pain.
I can tell you though, there is training that I've seen work very well. That is, the horse getting kicked spins & double barrels the other twice as hard & keeps doing it until the instigator backs off - that tends to knock them down a peg or few!
If I am out there when it happens I will make her move until she submits and gives me licking and chewing.
This exercise will be totally unrelated for the horse. Horses can't think rationally to link abstract ideas like humans can. They need instant, direct consequences, not random stuff done minutes - or even more than a couple of seconds - later.
If you feel the need for punishment, you need to do it immediately, AS the horse is doing Wrong. And you don't keep doing it for some proscribed time, but the punishment must stop when the Bad Behaviour stops, for a horse to be able to learn the desired lesson.
So... not that I suggest you punish the behaviour we are speaking of but IF
you were to do it in an effective manner, using 'make the horse move' it would be along the lines of... Horse starts to go at another with hooves or teeth, so you run the horse off forcefully. Maybe with a sting of a whip over her too. As she is moving away, she is not aggressing on her mate, so there is no need to continue punishment. If she comes back aggressively you again run her off. If she stays away or comes back meekly, you don't punish that.
Another reason I wouldnt punish this is, punishment is not well understood by an animal - or fair - if it's not consistent. You can't punish a behaviour sometimes but let it go at others(& of course you're not always out with them, let alone beside her to punish appropriately). The best she will likely learn is that she had better look out for you when you're close by.