There is only reason for this to have happened. One of the parents (most likely, the male) decided to begin culling the babies. In the wild, the parents will often select the weakest to kill (or eat!) which helps the strongest to survive, as there are fewer competitors for food or resources.
However, this can cause a problem if the only parent does not agree with the culling decision.
So I believe that Stevie was continuing to protect her young - but the tables had turned so she was now protecting them from their own father.
I removed them both immediately to the empty tank directly above the Infant Tank. I had planned on moving them anyway when the babies got a little larger, but truthfully, the babies can do well on their own as soon as they become free-swimming. Because there are no predators in the tank and the filter intake has a baby-proof sponge on it, there's no reason for the parents to keep them hidden among the plants and leaves.
Tomorrow: how the babies are doing on their own.