One possible reason could be a curse. Maybe someone in the past put a curse on the family, and this strange phenomenon is the result. Another reason might be some kind of genetic mutation that has never been seen before, which causes the baby's face to age prematurely.
Once there was a couple who adopted a baby. At first, the baby seemed normal. But as days passed, the baby's face started to change. It gradually took on the features of an old man. Its skin wrinkled, and its eyes became cloudy. The couple was horrified. They tried to find help but no one could explain it. Eventually, they were so scared that they left the baby in an orphanage, haunted by the strange transformation for the rest of their lives.
One reason could be a curse. Maybe the family was cursed generations ago, and this was the manifestation of that curse. For example, if their ancestors had wronged a powerful witch, she could have put a spell on the family line so that an abnormal baby would be born as a form of revenge.
One possible reason could be a magic spell. Maybe a wizard or a witch cast a spell on the boy for some reason, like revenge or as part of a test.
There was a family who welcomed a new baby into their home. But instead of the usual cute infant face, this baby had the face of an old man. It was a shock to everyone. The baby hardly ever cried like a normal one; instead, it made a low, raspy groaning sound. As days passed, the family members started having nightmares. In their dreams, the baby with the old man's face would crawl towards them with an eerie slowness. The mother couldn't bear it any longer and one day, when she went to check on the baby in its crib, she found the crib empty. All that was left was a withered flower, and from that day on, she was always looking over her shoulder, fearing the return of that baby with the old man's face.
Magic could be a reason. Maybe a wizard or a witch cast a spell on the boy accidentally or on purpose. Another possibility is a strange scientific experiment gone wrong in a fictional world. For example, if some mad scientist was trying to test a new gender - changing device.
One possible reason could be that there is some genetic factor in the family's ancestry that has led to a darker skin tone in the baby. Maybe there are hidden genes from a distant relative that have been passed on.
It could be magic. Maybe a wizard or a sorceress cast a spell on him for some reason, like revenge or just for fun.
She could be a vengeful spirit. Maybe she was killed near the tree and her spirit is now bound to it, seeking revenge on those who wronged her.
It might be a way to get her partner's attention in a very direct way. In a story, the wife may feel that her partner has been ignoring her or not taking her seriously enough, so she uses this rather extreme but effective method to make sure he focuses on her and her needs.
There could be some medical anomaly. Maybe there is some sort of abnormal connection between the digestive and respiratory systems that could cause air to come out in an unexpected way near the face. But this is highly unlikely.