As mandated reporters, when someone tells us that they are having violent thoughts towards their children, we are obligated to act. Imagine if the nurse had done nothing and the mother was then later violent with her baby.
As a mandated reporter, I always err on the side of reporting. In 10+ years as a pediatric nurse, I have seen CPS do too little too many times and do too much once. The time they did too much they took custody of a baby who had an unexplained femur fracture, I believe. That baby was later diagnosed with osteogenesis imperfecta. I acknowledge that it is absolutely horrible that his parents briefly lost custody of him when they did nothing wrong but it would be worse to leave all children with unexplained fractures in their parents' custody (potentially dangerous situations) while OI is ruled out. OI is very rare and to potentially send a child home to an abusive situation while it is ruled out is worse than to temporarily remove a child from what might be an abusive situation.
In this case, I don't think an OB is the appropriate provider to manage psychiatric symptoms (every article I've read on this case says that the mother expected her OB to prescribe her psychotropic medications) and since this woman's insurance is Medicaid and many mental health practitioners do not accept Medicaid, the fastest way to have her seen by a mental health clinician was likely to send her to the ER.