This is an Oedipus phenomenon. Just like a human baby, it feels safe only by holding its mother.
Because the little monkey always grabs the mother and refuses to let go when it wants to drink breast milk, and keeps drinking breast milk with its mouth. The little monkey is too small and timid, and cannot walk very fast. He is not very big, so because he is in his mother's arms all day long, he sometimes needs to drink milk, and sometimes his mother dare not let him go for fear that he will be bullied by other children.
When the environment changes, there is often a lack of food. When food cannot satisfy adult monkeys, in order to ensure the continuation of the population, the young will be abandoned first to reduce food consumption. When the cubs are sick, it will be difficult for the mother monkey to find food, and they are easily preyed on when avoiding natural enemies. The mother monkey has to abandon the cubs to avoid predators to ensure her own safety.