Fetal maldevelopment. The lens is a carefully arrayed structure with remarkable clarity. It requires precise biochemical and structural balance during development. If there is a disturbance (some known like rubella, inherited disease) and some unknown (simply accidental), then it must be evaluated for its visual disturbance potential. If you child has cataract see your pediatric ophthalmologist.
Several reasons. Congenital cataracts can be in one eye or both. They can be small or large. They can be isolated events or sometimes do to more serious problems. You should talk to your pediatrician.