There is a host of reasons why this happens.
If You read my hub where I have mentioned the development of mental apparatus, then you will be able to see where this may come from. Basically the superego becomes dorminant instead of the ego.
In other instances,certain family or enviromental factors make these children take the role of the parent, hence there is a role reversal.If a parent does nothing about it, it becomes almost irreversible, which is a selfish attitude of the parent.
In other instances, the parent narate stories which make the children feel responsible for what has happened. To understand what really happened, you need to consider each case but can not generalrise.