Babies have weak neck muscles and can't support the weight of their heads. If a baby is forcefully shaken, their fragile brain moves back and forth inside the skull. This causes bruising, swelling and bleeding. Shaken baby syndrome usually occurs when a parent or caregiver severely shakes a baby or toddler due to frustration or anger — often because the child won't stop crying. Shaken baby syndrome isn't usually caused by bouncing a child on your knee or minor falls.