There are three often inter-related reasons that people are forcibly displaced.
Persecution
Persecution is the systematic mistreatment of an individual or a group by another individual or group. This can take on many forms, including religious, national, social, ethnic, or political. Religious persecution is the biggest category.
War
Most of history’s refugees have been direct and indirect victims of war. Wherever there is armed conflict, people seek to escape death and conscription into armies by fleeing the conflict zone. Afghanistan had the largest number of refugees of any country in the world from 1981-2013, when it was eclipsed by people fleeing civil war in Syria.
Hunger and Climate
When a region faces extreme, prolonged drought, people flee to avoid starvation. Most of the 17 million refugees within Africa are fleeing starvation. The damage done by drought is made worse by war, making it impossible for farmers to farm. Some analysts fear that the number of refugees from droughts and other weather disasters is going to skyrocket to as many as one billion by 2050.