It's not the result of a bulging skull. Here (above) you can see the furrow moves independently of the skull. It seems to be brought about by a muscle that's much more subtle in adults.
My guess is that the furrow exists to make crying expressions read better. Nature really wants us to pay attention to crying infants.
Babies cry a lot broader than adults. They put their whole face into it. This (above) is an interesting picture. Look at the way the baby's chin is tucked up under his lip and the way the lip covers it.