You can say things like "how could this happen?," but at the end of the day things like this are a lot more common than we'd like to believe.
I work in healthcare and we place patients on 1:1 direct observation if they pose a danger to themselves or others. I can't tell you how many times a patient has fallen, gotten out of their restraints and started pulling at drains/tubing/oxygen etc. because the person in charge of watching them straight up wasn't paying attention.
Could there be an underlying conspiracy? Possibly. No convincing evidence I've seen thus far though. Negligence and incompetence still seems like the path of least resistance in terms of explanation.
Either way, good riddance.