Final answer:
The main trigger for shaking a baby, causing shaken-baby syndrome, is an infant's crying, with factors like stress and parental frustration contributing to the abuse.
Step-by-step explanation:
The primary trigger for shaking a baby, resulting in what is known as shaken-baby syndrome, is a baby's persistent crying. This syndrome encompasses a set of medical symptoms including brain swelling and retinal hemorrhage due to the forceful shaking or impacting of an infant's head. While many parents may simply be trying to soothe a baby's cries, the frustration resulting from their inability to calm the child can lead to this violent reaction. Contributing factors can include stressful environmental conditions such as a poor economy, unemployment, and general dissatisfaction with parental life. It's estimated that around 1,400 babies are affected by shaken-baby syndrome each year, leading to death or serious injury.