Unhealthy woman. Presumably because pushng the baby out at birth "pops" a brain artery. That would mean that the artery is itself sick since it is a highly abnormal result of birthing.
Immature Vessels. Infants born before 30 weeks of pregnancy are at highest risk for such bleeding. The smaller and more premature the infant, the higher the risk for ivh. This is because blood vessels in the brain of premature infants are not yet fully developed and are extremely fragile. The blood vessels grow stronger after 30 weeks of pregnancy.