I think the relevant point here is that police are only allowed to use proportional force.
Pushing someone to the ground, who is walking casually away with their hands in their pockets, is not a use of proportional force. This means it is an illegal and unnecessary use of force by the officer, which means he is fully culpable for any consequences of his actions. Since those actions led to the unintentional and unpredictable death of the person he pushed, he is likely to be charged with manslaughter - which is why the CPS are looking into pursuing a manslaughter charge against him.
Pushing someone to the ground, who is walking casually away with their hands in their pockets, is not a use of proportional force. This means it is an illegal and unnecessary use of force by the officer, which means he is fully culpable for any consequences of his actions. Since those actions led to the unintentional and unpredictable death of the person he pushed, he is likely to be charged with manslaughter - which is why the CPS are looking into pursuing a manslaughter charge against him.