The answer is natural causes and poor medical care.
The hospital started taking on more acutely ill children without having specialist neonatologists. All of the consultants were paediatricians who did neonatology on the side and spent most of their time with older children.
The hospital cut nursing staff, and regularly missed staffing standards, so babies who should have had one-to-one care weren't getting it.
The consultants did two ward rounds a week instead of two a day. So, much more than other units, this one was staffed by registrars and by trainees, many of whom were just rotating through for three months.
Errors which seem to have contributed to deaths Letby has been charged with include:
Not starting a blood transfusion on time / at all.
Setting ventilation settings too high so that children's lungs were over-inflated and their hearts put under pressure.
Long delays offering antibiotics, or offering the wrong antibiotics.
Failures at intubation.
We see lots of understaffed, dirty, penny pinching NHS units experience scandals, with higher death rates, even among babies. When experts have found these errors in the records, and the doctors treating these children weren't experts, why is it hard to believe that they simply, sadly, got things wrong?