There are studies from 15 years ago about spaying at the wrong time during a cycle and ending up with a permanently aggressive female. But even with regular spay/neuters a great deal of hormonal change can cause many dogs to react. It is biological but still important to stay on top of behaviors so they don't become common.
As for the fight...spayed, not spayed, neutered or not, it is not uncommon for dogs who have always been fine to have one fight and never get along again.
There are some worrisome new research about reactive and fearful behaviors with neutered dogs.
If anyone is interested, glad to share the links (too tired to fetch them tonight. Back to my normal backing up posts with links so that anyone wanting to read more can.)
