Parents need to protect children from harm during this stage. They are at risk for poisoning, falling down steps and off furniture and getting an electric shock. Sometimes when a parent rescues the child from danger, the child becomes angry or aggressive. Often a child goes right back to the danger, making her seem defiant. That can make the parent lose patience or become outraged and hit the child.
Sometimes a child is well behaved around others and only tests limits when his parents are around. This is because he feels most comfortable with them. However, because it looks like he is deliberately disobeying them, his parents may overreact.