Hi spottedpupmom and thanks for posting your question.
Based on podcasts I have done with tenderfoot training
(one of the experts on our board) and my own experiences with many dogs, dogs almost never go from Zero to aggression. You suggest there is no trigger, but I would be willing to bet that if you were able to carefully watch the body language of your two dogs, that you would see the warning signs of aggression. These may include direct eye contact, raised hackles (the hair on the neck and back) a low growl, showing of teeth...
If you catch these signs early (I realize not always easy in an active household) you should be able to halt the aggression before it happens by redirecting the dogs in some way.
We did get a new puppy several weeks ago, but he lives inside and outside, and hasn't even been in the house during many of the fights, and hasn't been involved in any of them.
The smell of this new puppy might also be the trigger here.
I definitely agree with LL that a vet visit is also a good idea here to rule anything out.
I'd be very curious to hear what other members suggest.