Where do kids learn it?

Last week Abigail wrapped some string around Brooklyn's neck and was running around her. I quickly went and took it off and told Abigail we do not put strings around other people's necks. Then today I was feeding Xavier when I hear a muffled cry from Brooklyn. It is her hurt cry. So I quickly put the baby down and run into their room. Brooklyn was laying on the bed and had blankets all over her. She had a pillow on her head and Abigail was sitting on the pillow. I quickly pull Abigail off and put her in time out. Brooklyn wanted me to hold her, but not until after she stripped all of her clothes off. During that time she is screaming of fear and pain and Xavier is screaming because he has not finished eating yet.

I just wonder where kids learn how to attack each other like that. I have never taken a pillow and put it on top of one of my kids heads and sat on it. Who knows. I just hope I can get this phase to go away.