No good explanation here, but experience. When hormones surge, and sometimes because something is worrying the duck (I presume), I will have one call, and call an alarm call repeatedly.
I check around, make sure the pen is secure and everyone is in, and then don't worry much more about it.
I reckon she could have a tummy ache, or could smell a predator, something like that. Obviously she is trying to tell me something, and I always respect that in a duck. Just because I don't see it doesn't mean there is not a threat.
Now Einz is usually my alarmist. And she hates getting used to new ducks. She screamed for three weeks last fall when we introduced a new buff trio.
She is much calmer now.
So just make sure they are safe with good food and water, look around, and you may discover later what is bothering her.