Why did the Pashtuns persecute the Hazara?

This is actually a pretty complicated question. There are many different ethnic groups in Afghanistan. At times they resemble the Indian caste system. Amir is Pashtun and Hassan is Hazara. Pashtuns are Sunni Muslims and Hazaras are Shi'a Muslims so religion and ethnicity are major ideas that conflict. Afghanistan has always had warring cultural factions. At this point in history, Pashtuns are the dominant socio-political force.