Here are some of such reasons:
1. physical punishment - parents can sometimes beat or smack their children for doing something wrong even though technically that is illegal now
2. emotional abuse - parents may neglect their children and not be around for them when they need them
3. financial issues - sometimes parents have a lot of children and not have the means to support them
4. alcohol/drug abuse by parents - which may cause them to become aggressive
5. stress at work - which may make them agitated