edA monkey stealing a human baby is an incredibly rare occurrence, but it has happened before. There are a few possible explanations for why this might happen.
First, the monkey may have been unusually aggressive due to lack of food or feeling threatened by the humans around them. Second, the monkey may have seen the human baby as a substitute for a young monkey, and therefore may have wanted to protect or take care of the baby. Third, the monkey may have mistaken the baby for food, since babies, like young animals, often move unpredictably and make noises that monkeys may interpret as a call for help.
Such behaviour in monkeys is extremely rare, and is most likely a combination of instinct and learned behaviour.