It has been recalled through a series of memoirs that Mostafa Khomeini was in prison for a while, and when he was in Qom, the establishment was extremely sensitive towards him and was against him.
Even going to prison did not affect Mostafa spirit for the revolution stuggle under Imam’s leadership.
He was not a person who would lose resistance in jail, but prison made him stronger and more powerful every day, so they were forced to exile him.
(Extracted from a series of memoirs about Imam)