Answer: After the victory of the Islamic revolution of Iran, the western countries, especially USA, organized facilities to attack the new Iran's Islamic system and began to establish the bases of the opposed Iranian outside of the country.
These opposed countries did not meet the Iranian legal desires to establish a divine system and the returning of Shah and the blocked properties which were 22 Billion Dollars.
The hostile deeds of USA on Iran, especially its accepting Shah , together with the un-announced news of Bazargan meeting with Berjhineski, the national security counselor of the white house in Al-Jazeera, triggered people's anger. All of these issues were the cause of some revolutionary students' movement to capture USA Embassy in the13th of Aban 1358.
They could imprison the American riflemen. The secret documents appeared in USA ministry (which are published more than 50 volumes), indicated USA's spying and interventions in Iran and other countries of the world.
After the capture of USA's spying center, Imam Khomeini supported the students' revolutionary act and considered it as a greater revolution than the first one.