How come some people get crossed eyes?

Childhood, acquired: Crossed eyes are out of alignment inwards (esotropia), outwards (exotropia) or vertical (hypertropia). Vision holds the eyes aligned so poor vision will cause one eye to drift if it happens in childhood. Adults will lose alignment when their childhood stability gets disrupted or when they have trauma to the eye muscles or the eye muscle nerves. See a strabismologist ophthalmolgist for answers.