The same kind of thing that's been going on for a very long time - people are being cruel to their children.
The first state intervention between parents and children happened in 1889, when the first act aimed to prevent cruelty to children was passed. Before that date hundreds of thousands of children were subjected to the most appalling cruelty - the situation was far worse than it is today. Since then there have been many amendments to the act, and many new acts, all with the same aim - to give the state greater powers to intervene in the relationship between parents and their children if things go wrong.
On average one child is killed by their parent or carer every week in England and Wales, and the children most likely to die a violent death are babies under 1 year old. They are four times more likely to be killed than the average person.
The psychology behind child killings would fill a dozen forum threads, it's so complex, and at times incomprehensible. It will come as no surprise however, to hear that many child abusers were themselves subjected to abuse and cruelty when they were children.