The reason why babies pee when you take their diaper off is because the bladder is stimulated by the sudden cool air on the babies genitals.
The sudden cool air that reaches the baby's genitals triggers a reflex that causes the baby to pee or urinate when the diaper is removed.
While baby girls can urinate when you take their diaper off, it's very common in baby boys due to their anatomy.
Babies have no bladder control and so any exposure to the cold air that triggers the reflex will lead to the baby peeing once you take their diaper off and they cannot control it.
A baby's bladder control is not the same as older children or adults as their bodies have not yet learned to regulate when and how much urine is released.
The sudden cooler air stimulates the bladder to contract and results in urination and is a common phenomenon known as cold diuresis.
When changing a baby boys diaper and to prevent them from peeing on you when you take their diaper off you can place a wet cloth or baby wipe across your baby's belly at the diaper line before you change them.