Abuse can happen for the several reasons below:
1. If the parents are attached with the kids and have a disgust if the kid is not performing well enough up to their expectation then they might use harsh words or physically abuse them.
2. Anger is one letter away from the word Danger. Some of us think that we cannot get things done unless we get angry. Why do we always get angry at people who are dependent like children. This instills fear in them and they start feeling inferior, and will not improve but in turn deteriorate.
3. Kids might decide to get even with parents when they get older.
4. Parents fight among themselves and sometimes take it out on the child.
5. Parents are not aware of the effect of using negative words. Whether we call a child stupid or smart, either way he will get spoiled. The reason we call him ‘smart’ is so his ego gets encouragement; and if we call him ‘stupid’ then it will have a reverse psychological effect on him. If you tell a good person he is foolish and mad fifty times or so, then he will begin to doubt himself, ‘May be I really am foolish!’ In thinking this way, he will become mad.
6. Parents keep comparing one child with the other, but every child is different. So he might lose confidence in himself and not make right choices in life, by trying to please parents.
Please see this link on how to bring up kids in general: