One cause, the industrial revolution & new mass manufacturing developments. Farm people moved from clean open country to expensive & dirty crowded cities looking for more lucrative work opportunities. These conditions made it very difficult for couples to raise large healthy families. Tiny tenement apartments & poverty conditions (lack of refrigeration; adequate indoor plumbing & sanitation) led to deadly disease problems. Infant & child mortality rose significantly until living conditions & income improved.
The potato famine & landlord/political abuses in Ireland also contributed to major birth rate drops in the US & Europe. Europeans immigrated to escape that hardship & found even worse problems here. Often men & husbands disappeared, looking for work, or just inexplicably, leaving single women unable to marry & wives unable to have more children. Or care for the children they had adequately. The ratio of available men to women dropped significantly causing less children to be born. And as shoup says, the Civil War had absolutely devastating effects on population growth. Esp. in the south. An entire generation was nearly lost. Future generations as well. Tragic times.