Authority: A Driving Force Behind the Reformation

What was one driving force behind the Reformation?
Joel Beeke explains in this video that the Protestant Reformation began as part of Martin Luther’s opposition towards the selling of indulgences to allegedly “release” souls from purgatory.
This abuse from the Roman Catholic Church contributed to one of the driving forces behind the Reformation — the issue of authority. Who is in charge of our lives? Is Scripture sufficient for everything or is the pope a second authority source that is of equal value to Scripture? If there is conflict between the two, which do you believe?
The fact that the pope seemingly had equal authority to Scripture forced the early Reformers to confront the issue of the authority of Scripture head-on.