Reasons: physical abuse(leaving attended marks on your child like bruising or marks on purpose) physical neglect(leaving your child dirty or not giving the proper amount of shelter or food) , sexual abuse(sexually using a child to stimulate oneself), mental abuse(making the child feel lower, not good enough, or unwanted ext...) those are examples of why someone would take a kid from a home. Reasons for knowing, I'm currently Taking classes to become a social worker.
Child services can take away your kids if they get reports or have proof of any form of abuse like physical, mental and sexual abuse. They can also take custody of your kids if there is proof of neglect like medical or physical neglect. Visit http://www.cwla.org/childwelfare/fg02.pdf for more information.