Al-Hajjaj hated the Ahlul Bayt and their followers very much. So, he filled the prisons with Shias. They had no fault but they loved Muhammad's family.
Al-Hajjaj was unjust. People hated him. They asked Abid al-Malik to remove him from office. Abid al-Malik refused because al-Hajjaj wanted to strengthen the Umayyad rule with fire and sword.
Al-Hajjaj was wicked. He sent people to fight and occupy lands. He forced them to join the invading armies. He wanted them to get the booty of the conquered lands. In the meantime, he wanted to get rid of them.