Final answer:
Hosni Mubarak was imprisoned after resigning as Egypt's President due to his involvement in ordering violence against protesters during the Arab Spring and for alleged theft of state funds.
Step-by-step explanation:
Egypt's former President Hosni Mubarak was imprisoned after he resigned from office largely due to the Arab Spring protests of 2011 which emerged in reaction to government corruption, the lack of democratic reforms, and political oppression. On February 11, 2011, following massive protests across Egypt, Mubarak announced his resignation. Subsequently, he was put on trial and convicted for his role in ordering government forces to harm and kill protesters during the demonstrations. The outcry for justice from the Egyptian people also included accusations of Mubarak having stolen billions of dollars from the state and depositing them into his own accounts, further adding to the charges against him.