It happens because dogs are dogs, not people. They think and behave as dogs do, not as we do.
There may be triggers which instigate the troubles, but the best one can do if you find triggers is to avoid exposing the dog to those things which cause the abberant behavior.
Two females fighting is probably a dominance issue. One reason may be because your dogs do not receive leadership signals from you which they understand and so, if they don't feel you are the Pack Leader, they will fight to secure their place as Leader because pack animals MUST have a leader.
You are doing your best now to manage the behavior as best you can. That's good. Keeping them apart for now, until you and a behaviorist can begin working on the issue and YOU can learn to establish your leadership in the eyes of the dogs, is a good thing and smart thing to do.
But please, get some help and like Andie said; don't let anyone tell you that you must get rid of one of the dogs until you have given them a chance to learn proper behavior and pack behavior and given yourself a chance to understand the dynamic and how to handle it.
Because "trainers" find it difficult to deal with aggressive dogs and it is frightening to most of them, the easy answer is always to get rid of the offending dog (aggressive one) and/or put one of them down. I say that's a cop out!!
Give the dogs a chance to learn, be reconditioned and their behavior modified before even considering giving your dog(s) up or worse. That should be your LAST option once you have exhausted all others.
Get some qualified professional help. Call behaviorists, tell them the problem and ask if they will be willing to work with you to RESOLVE the issue, not just get rid of the dog because it is easier.
I have two current clients in the exact same boat as you and your dogs. It is frightening and very upsetting to have to be breaking up nasty blood letting fights between two dogs, but the owners are determined to help their dogs; not give them away or destroy them, and I am willing to devote as much time, energy and knowledge available to me to try and help them resolve this problem without the need to get rid of their beloved pets.
I am making progress with both clients. Yes, it is slow and frustrating and they are a bit astounded by the cost, but the love their pets and want only to help them. Find someone who believes the way you do about your pet and don't let them convince you there is no hope here. There is always hope. Where there is a will, there's a way and don't ever forget that.
Best of luck and keep us posted.
In the meantime, you might want to get a copy of "The Dog Listener" by Jan Fennell and begin working on your leadership skills so that the dog's will stop vying and fighting for leadership between themselves. If you are established and trusted in their minds as leader, they will settle into their roles without them feeling a need to compete/fight for a position as leader.
Patience, love and hard work will get the job done. Don't give up yet.