The most common reason is she hears, smells, or saw something that she wasn't sure of and is trying to let everyone know. It could be anything from an animal or something outside, something in the walls, the heat clicking on (that one can be smell and sound), etc.
If she was in the wild, that would be warning the herd. They continue to have those instincts, regardless of whether there is a herd to warn or not! Anything and nothing can set them off, as Amethyst mentioned above. I just learned to tune it out.
I have a 9 yr. Old male screamer! I think it's more instinct for my boy. Sometimes he will scream every night, and then go a month or more and I never hear him. It is truly unnerving to hear sometimes. I started counting when he screams and it's almost always 7 in a row, lol.