Seriously, it's not worth debating or analyzing the reasons "why" the dam is aggressive now and not previously.
Simple fact is -- she is. Simple fact is -- it's nature, normal, and par for the course. Simple fact is -- keep her separated.
Starting now, segregate her from ALL of your other dogs, for 100% of the time. This means separate, safe, and secure living quarters for the dam (including the pups too, when they arrive). She will also require solitary feedings, walks, yard time, play time, socialization time etc. Use barriers, crates, x pens, and / or closed doors as a means to do so. At least until the puppies are ~ 8 to 10 weeks old and they leave for their new homes. THEN, and only then you can begin to gradually re-introduce her to the rest of your dogs.
My guess is, that may seem like a lot of extra work to you. But these steps must be taken because the safety of your adult dogs AND THE PUPS is at stake here.