What Causes Violent Behaviour In A Child?

Hello there
I have a 5 yr old son who has a step mother with a 2 month old. She said he hit the baby on her soft spot and now almost had to go to the children's hospital and says the doctor says her brain has swollen (no proof of any of it) and there's been real damage done. Could my son have hit her that hard? He has never hit or showed any signs of hitting my baby.... Thanks